ask the teacher they know all about that stuff
y = x^2 + x
z = y^2 + y
substitute y = x^2 + x into z = y^2 + y
z = (x^2 + x)^2 + (x^2 + x)
z = x^4 + 2x^3 + x^2 + x^2 + x
z = x^4 + 2x^3 + 2x^2 + x
Answer is
x^4 + 2x^3 + 2x^2 + x
