Question

12 – (3х² + 5х) + (–7х + 3х²) = 0

Answer 1

12-3х(2)-5х-7х+3х(2)=0
12-12х=0
-12х=-12
х=1

Answer 2

$\displaystyle\bf\\12-\Big(3x^{2} +5x\Big)+\Big(-7x+3x^{2}\Big )=0\\\\12-3x^{2} -5x-7x+3x^{2} =0\\\\\Big(\underbrace{-3x^{2} +3x^{2}}_{0} \Big)-\Big(\underbrace{5x+7x}_{12x}\Big)+12=0\\\\-12x+12=0\\\\-12x=-12\\\\\boxed{x=1}$