Note that c is the hypotenuse of the blue triangle, and that the Pyth. Thm. states that (length of one leg)^2 + (length of the other leg)^2 = (hyp)^2.
Therefore, (hyp)^2 = c^2 = [2sqrt(x^2+3x)]^2 + 3^2, or = 4(x^2+3x) + 9, or = 4x^2 + 12x + 9 = (2x+3)^2 Taking the sqrt of both sides, c = plus or minus (2x+3). Eliminate -(2x+3) because the middle term of the square of this would be negative, in conflict with the given +12x.
