If twice a number is divided by 7, the result is -28

If twice a number is divided by 7, the result is -28.

The phrase a number means an arbitrary variable, let's call it "x".
Twice x means we multiply x by 2: 2x

Divide this by 7: 2x/7

We set this equal to -28, and we have our algebraic expression:
2x/7 = -28