powerprep question..help ..its irritating me

Since we want the value of m + n, let's see what equations we can come up with.

m = z + 4

n = e + y

And if we add them together, we get:

m + n = z + 4 + e + y

Well, so far we don't have enough information - we need to find values for e, y and z.

So, let's grab some more equations:

4 + y = -5 (so y = -9)

e + z = 10

Great! Let's just rearrange our equation to:

m + n = y + (e + z) + 4

and now we just sub in:

m + n = -9 + (10) + 4

m + n = 5