Evaluate the Expression: Finding Solutions for ((8by)^3)^y + (3^x)^a

First, we'll carefully open the parentheses, using two laws of exponents:

The first law is the exponent law that applies to parentheses containing multiplication of terms:

$(c\cdot z)^n=c^n\cdot z^n$which essentially states that when there is an exponent acting on parentheses containing multiplication between terms, when opening the parentheses the exponent will apply separately to each of the multiplication terms inside the parentheses.

The second law we'll use is the power of a power law:

$(c^m)^n=c^{m\cdot n}$which essentially states that when applying an exponent to a term that is already raised to a power (in the above form - inside parentheses for good order, but generally - also without the parentheses), we can interpret this as multiplication between the exponents within the exponent notation.

Let's return to the problem and first deal with the two parenthetical terms in the overall sum separately-

1. The first from left to right is:

$\big((8by)^3\big)^y=(8^3b^3y^3)^y=(8^3)^y(b^3)^y(y^3)^y=8^{3y}b^{3y}y^{3y}$where we used the first law above twice, first for the inner parentheses and then for the remaining parentheses, but we did this carefully because the terms in the multiplication within the parentheses are raised to powers, so we did this using additional parentheses, then we applied the power to the power (effectively opening the parentheses) using the second law above.

2. The second from left to right is:

$(3^x)^a=3^{xa}$where we applied the power to the power using the second law above.

Going back to the problem, we got:

$\big((8by)^3\big)^y+(3^x)^a=8^{3y}b^{3y}y^{3y} +3^{xa}$where we used 1 and 2 that we noted above.

We got the most simplified expression, so we're done.

Therefore, the correct answer is A.

Important note:

It's worth understanding the reason for the power of a power law mentioned above (the second law), this law comes directly from the definition of exponents:

$(c^m)^n=c^m\cdot c^m\cdot\ldots\cdot c^m=c^{m+m+m+\cdots+m}=c^{m\cdot n}$where in the first stage we applied the definition of exponents to the term in parentheses and multiplied it by itself n times, then we applied the law of exponents for multiplication between terms with identical bases mentioned above and interpreted the multiplication between the terms as a sum in the exponent notation,

Then we used the simple multiplication definition that states that if we connect a number to itself n times we can simply write it as multiplication, meaning:

$m+m+\cdots+m=m\cdot n$and therefore we get that:

$(c^m)^n=c^{m\cdot n}$