What you want to first is substitute the numbers in for the variables in the equation
Since b= -5 , we will plug it in for -b
-5 + c = -13x
Now plug in -8 for c
-5 + (-8) = -13x
Add the numbers on the left side, which gives you -13.
-13 = -13x
Lastly, divide both sides by -13x, which will give you 1.
So x = 1
