I assume you want the roots.

Simplify and use quadratic formula:

12a² - 13a - 35 = (12a²-28)(15-35)

12a² - 13a - 35 = (12a² - 28)(-20)

12a² - 13a - 35 = -240a² + 560

252a² - 13a - 595 = 0

a = (-(-13)±√((-13)² - 4(252)(-595)))/(2*252)

  = (13±√599929)/504

Approximately these roots are a = 1.5626... and a = -1.5110...