a) $3x^2 – 3x(x – 2) = 36$.
<=> $ 3x^2 – (3x.x – 3x.2) = 36$
<=> $ 3x^2 – 3x^2 + 6x = 36$
<=> $ 6x = 36$
=> $x = 6$
b) $5x(4x^2 – 2x + 1) – 2x(10x^2 – 5x + 2) = -36$.
<=> $5x.4x^2 + 5x.(-2x) + 5x.1 – (2x.10x^2 + 2x.(-5x) + 2x.2) = -36$
<=> $20x^3 – 10x^2 + 5x – (20x^3 – 10x^2 + 4x) = -36$
<=> $20x^3 – 10x^2 + 5x – 20x^3 + 10x^2 - 4x = -36$
<=> $x = -36$