With the facts you got here, you can make the following equation:
x*(x+2)=24
x^2+2x=24
x^2+2x-24=0
Then you can use the following formula (which you might have seen before):
x = (-b +/- sqrt(b^2-4ac))/2a
where
ax^2+bx+c=0
(-2 +/- sqrt(4+96))/2
(-2 +/- 10)/2
x_1 = 4
x_2 = -6
