Solve the Equation: Find the Missing Factor in 7 × ? = -21

Question

Fill in the missing number:

$7\cdot?=-21$

00:00 Find the unknown

00:04 Positive times negative always equals negative

00:10 Therefore the unknown is negative

00:16 Now let's find the unknown

00:22 This is the unknown, now let's add its sign

00:26 And this is the solution to the question

Remember the following law:

$(-x)\times(+x)=-x$

Let's think about which number we need to multiply by 7 in order to get 21:

$7\times3=21$

Now let's put the numbers together with the appropriate sign as written in the law above as follows:

$+7\times(-3)=-21$

$-3$