(а+6)^2- a(a-3) = (a+6)(a+6) - a(a-3) = a^2 + 6a + 6a + 36 - a^2 +3a = a^2 + 12a + 36 - a^2 + 3a = 15a + 36
(а+6)^2- a(a-3) = (a+6)(a+6) - a(a-3)
= a^2 + 6a + 6a + 36 - a^2 +3a
= a^2 + 12a + 36 - a^2 + 3a
= 15a + 36