The Domain of an Algebraic Expression: More than one unknown

What will be the result of this algebraic expression:

$8a-b(7+a)$

if we ascertain that:

$a=50,b=0$

Let's insert the given data into the expression:

8*50-0(7+50) =
400-0*57 =
400-0 =
400

$400$

What will be the result of this algebraic expression:

$8a-b(7+a)$

if we place

$a=2,b=\frac{1}{3}$

$13$

What will be the result of this algebraic expression:

$8(x-7)+4(6-2y)$

if we place

$x=0,y=-1$

$-24$

What will be the result of this algebraic expression:

$8(x-7)+4(6-2y)$

if we place

$x=8,y=5$

$-8$

What will be the result of this algebraic expression:

$8a-b(7+a)$
if we place

$a=-\frac{1}{2},b=\frac{2}{13}$

$-5$

What will be the result of this algebraic expression:

$8(x-7)+4(6-2y)$

if we place

$x=7.1,y=\frac{5}{8}$

$19.8$