1) $$18xy - 6x^2 = 6x(3y - x)$$.
2) $$15a^6 - 3a^4 = 3a^4(5a^2 - 1)$$.
3) $$4x - 4y + cx - cy = 4(x - y) + c(x - y) = (x - y)(4 + c)$$.
Ответ: 1) $$6x(3y - x)$$, 2) $$3a^4(5a^2 - 1)$$, 3) $$(x - y)(4 + c)$$