Charcoal, 12 10 bytes
IΣEN×ιX⊕ι²
Try it online! Link is to verbose version of code. Explanation: \$ ( \sum x )^2 = \sum x^3 \$\$ ( \sum_1^n x )^2 = \sum_1^n x^3 \$ so \$ ( \sum_1^n x )^2 - \sum_1^n x^2 = \sum_1^n (x^3 - x^2) = \sum_1^n (x - 1)x^2 = \sum_0^{n-1} x(x + 1)^2 \$.
N Input number
E Map over implicit range i.e. 0 .. n - 1
ι Current value
⊕ Incremented
² Literal 2
X Power
ι Current value
× Multiply
Σ Sum
I Cast to string
Implicitly print