How to do work out C+4=-24

I assume you want the value for C. When you have an equation, as long as you do the same thing to each side, it remains true. So,

C+4 = -24

You want C by itself, so if you subtract 4 from both sides, you have

C+4-4 = -24-4

4-4=0, and of course, C+0 is just C, yo now you have

C = -24-4
or,
C = -28