1)15a^2x-18ax^2= 3ax(5a-6x)
2).3a^3-12a^2+6a= 3a(a^2-4a+2)
3).15x^4y^3-5x^2y^2+10x^2y=5x^2y(3x^2y^2-y+2)
-4x(x+2)+4(x+1)²=5x-21
-4x²-8x+4(x²+2x+1)=5x-21
-4x²-8x+4x²+8x+4=5x-21
-4x²-8x+4x²+8x-5x=-21-4
-5x=-25
x=-25:(-5)=25:5
x=5
-4*5(5+2)+4(5+1)²=5*5-21