4) 36x^4 - 25y^2 = (6 x^2 + 5y)*(6x^2 - 5y)
5) 2x^2 - 2(x- 5)^2 = 2x^2 - 2x^2 + 10x - 50 = 10x - 50 = 10(x - 5)
6) 18 + 3(x + 1)^2 = 18 + 3x^2 + 6x + 3 = 3x^2 + 6x + 21 = 3(x^2 + 2x + 7)
7) 6(x - 5)^2 + 25x = 6x^2 - 60x + 150 + 25x = 6x^2 - 35x + 150