Show that $3^3-2^3=3(2)^2+3(2)+1$

For $x\in\mathbb R$ prove by starting with the left-hand side and transforming it that $(x+1)^3-x^3$ simplifies to $3x^2+3x+1$ In your final line, use the appropriate symbol to indicate that this is an identity.

Hence determine $21^3-20^3$