Question

3x^2-15=0, x^2+7x=0, 12x^2-5x-2=0

Answer 1

$3 {x}^{2} - 15 = 0 \\ {x}^{2} - 5 = 0 \\ x = + - \sqrt{5}$

${x}^{2} + 7x = 0 \\ x(x + 7) = 0 \\ x = 0 \\ x = - 7$

$12 {x}^{2} - 5x - 2 = 0$
D = 25+96=121
$x = \frac{5 + 11}{2 \times 12 } = \frac{2}{3} \\ x = \frac{5 - 11}{2 \times 12} = - \frac{1}{4}$