With a math problem : / show
x^2 -13x = -36

X^2 -  13 x = -36 
add 36 to both sides

x^2 - 13x +36 = 0
figure out the factor
(x-4)(x-9) = 0
so (x-4) or (x-9) equals 0
so solution is
x-4=0 add 4 to both sides and get x = 4
x-9=0 add 9 to both sides and get x=9
x={ 4, 9}

Google it that might work