Tìm x

a) 5x2 – 5x(x – 2) + 30 = 0
b) 5x(12x + 7) - (3x + 1)(20x – 5) = -100
c) (x + 3)(x – 3) – x(x – 2) = 0
d) (x+50)2 + (x + 4)(x - 4) = 0