а) $$x^2 - 5x = $$ $$x(x-5)$$
б) $$y^2 - 25 = $$ $$(y-5)(y+5)$$
в) $$a^2 + 12a + 36 = a^2 + 2 \cdot a \cdot 6 + 6^2 = $$ $$(a+6)^2$$