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

Let's remember the law:

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

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

$7\times3=21$

Now let's put the numbers together with the appropriate sign as written in the law above, and we'll get:

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