Question

x(3x+19)+12=3(4+7x-x)²

Answer 1

3x^2 + 19x + 12 =  3 (4 + 6x) ^2

3x^2 + 19x + 12 = 3(16 + 20x + 36 x^2)

3x^2 + 19x + 12  = 48 + 60x +  108x^2

105x^2 + 41x + 36 = 0

Aplica formula cuadrática y ya :)