Question 251309
First multiple out the equation:

x(x-3)=40  so:  x^2 - 3x = 40

now set the equation zero:  

x^2 - 3x -40 = 0  now you can factor the equation:

(x-8)(x+5) = 0

so either x-8 = 0 or x+5 = 0

so x is either 8 or -5...try them both back into the orginal equation

8(8-3)= 40 
8(5)=40
40 = 40   yes

-5(-5-3) = 40
-5(-8)=40
40 = 40  yes

so the anwser is both x = 8 and x = -5