Yes, you are correct to use proportions because they are similar triangles.
The answer is 7. You need to use BIDMAS - Brackets Indices Division Multiplication Addition Subtraction
Note the equal sign. What you do to one side, you do to the other.
Isolate the variable. Divide 4 from both sides
(-52)/4 = (4m)/4
m = -52/4
m = -13
-13 is your answer for m
hope this helps