Simple....
we have:
\(\frac{-m}{20}\)+4=7
We must first isolate what we want to find...or m......
So...we must..
\(\frac{-m}{20} +4=7\)
-4 -4
Leaving you with...
\(\frac{-m}{20} =3\)
Thus, your answer isolate the variable. (Subtract 4 from both sides.)