Answer:
The expression -21 x (x + 28) can be simplified by using the distributive property of multiplication over addition.
Using the distributive property:
-21 x (x + 28) = -21x^2 - 21 x 28
Multiplying:
-21x^2 - 21 x 28 = -21x^2 - 588
So the simplified expression is equal to -21x^2 - 588.