50a²/(5a + 10) - 10a = 50a²/(5a + 10) - 10a(5a+10)/(5a + 10) = (50a²-50a²-100a)/(5a + 10) = -100a/5(a + 2) = -20a/(a + 2) = -20*3/(3 + 2) = -20*3/5 = -4*3 = -12