What is x if 72/(3x+3) -x=3?

First, we'll factorize by 3 the denominator of the ratio
72/(3x+3) = 72/3(x+1).

Then, we'll divide by 3 both
numerator and denominator of the ratio:

72/3(x+1) =
24/(x+1)

We'll have to multiply both sides by the
denominator x+1.

24(x + 1)/ (x + 1) - x(x+1) =
3(x+1)

We'll simplify and we'll move all terms to one
side:

24 - x(x+1) - 3(x+1) =
0

24 - x^2 - x - 3x - 3 =
0

We'll combine like terms:

-
x^2 - 4x + 21 = 0

We'll multiply by
-1:

x^2 + 4x - 21 = 0

We'll
apply quadratic formula:

x1 =
[-4+sqrt(16+84)]/2

x1 =
(-4+10)/2

x1 = 3

x2 =
(-4-10)/2

x2 = -7