math

x^2 - 3x  = 88    subtract 88 from both sides

x^2 - 3x  - 88 = 0   factor

(x - 11) ( x + 8)  =  0     setting both factors to 0 and  solving for x and x = 11  or x  =  - 8

Or after substracting 88 you could apply the formula for second degree equations, if you're not good at factoing. You have to be sure you have arranged well all the numbers.

\(x = {-b \pm \sqrt{b^2-4ac} \over 2a}\)

Where, in this particular case, a=1, b=3, and c=(-88). It will give you the two results after splitting the equation in the sum and substraction due to the "add AND/OR substract" sign.