a) $5x^2-5x(x-2)+30=0$
$⇔ 5x^2-5x^2+10x+30=0$
$⇔ 10x=-30$
$⇔ x=-3$
b) $5x(12x+7)-(3x+1)(20x-5)=-100$
$⇔ 60x^2+35x-(60x^2-15x+20x-5)=-100$
$⇔ 60x^2+35x-60x^2-5x+5=-100$
$⇔ 30x+5=-100$
$⇔ 30x=-105$
$⇔ x=-\dfrac{7}{2}$
c) $(x+3)(x-3)-x(x-2)=0$
$⇔ (x^2-9)-x^2+2x=0$
$⇔ 2x-9=0$
$⇔ 2x=9$
$⇔ x=\dfrac92$
d) $(x+50)^2+(x+4)(x-4)=0$
$⇔ x^2+100x+2500+x^2-16=0$
$⇔ 2x^2+100x+2484=0$
$⇔ x\in \varnothing$
