3b+bc+3ac+9a = (3b+bc)+(3ac+9a) = b(3+c)+3a(c+3) = (c+3)(b+3a)
a²y²-x⁶ = (ay)²-(x³)² = (ay-x²)(ay+x²)
c²-6c+9-4x² = (c²-6c+9)-4x² = (c-3)²-(2x)² = (c-3-2x)(c-3+2x)
xy²+y+x²y+x+4xy+4 = (xy²+y)+(x²y+x)+(4xy+4) = y(xy+1)+x(xy+1)+4(xy+1) =
= (xy+1)(y+x+4)