Question 1
(x-3)(x^2+4x+5)=x(x^2+4x+5)-3(x^2+4x+5)=x^3+4x^2+5x - 3x^2-12x-15=
x^3+(4-3)x^2+(5-12)x-15=x^3+x^2-7x-15.
Question 2
(x+1)(2x-1)(x+3)=(x+1)(2x(x+3)-(x+3))=(x+1)(2x^2+6x-x-3)=(x+1)(2x^2+(6-1)x-3)=(x+1)(2x^2+5x-3)=
x(2x^2+5x-3)+(2x^2+5x-3)=2x^3+5x^2-3x+2x^2+5x-3=2x^3+(5+2)x^2+(-3+5)x-3=
2x^3+7x^2+2x-3.