discrete mathematics - Using induction to verify the formula for a summation $sum_{k=1}^n k^2 = frac{n(n+1)(2n+1)}6$

Problem 4. use the principle of induction to verify:
$$\sum_{k=1}^n k^2 = \frac{n(n+1)(2n+1)}6$$

base case is obviously easy, but I don't know how to prove the inductive case