That is a quadratic equation in which:
a = 2 b = 5 and c=-12
We use the quadratic formula:
x = (-b +- sqr root (b^2 - 4*a*c)) / 2*a
x = (-5 +- sqr root (25 - 4 * 2 * -12)) / 2 *2
x = (-5 +- sqr root (25 +96)) / 4
x = (-5 +- 11) / 4
x1 = 6 / 4 = 1.5
x2 = -16 / 4 = -4