You essentially have, on the right side, x + -1(-34) , which equals   x + 34 right? (subtracting by a negative can be written as addition)

then subtract 34 from both sides and you are left with x = 90.

124=x-(-34) can also be written as 124=x+34 since subtracting a negative number is the same as adding a positive number.

we now want to isolate the x (get the x on the side by itself).

In order to do this, we will subtract 34 from both sides. 

This gives us 124-34=x+34-34

The + and - 34 on the right side cancel out and all you are left with is x. 

124-34 is 90 so the answer is 90=x OR x=90.