when a certain spring is stretched by an amount x, it produces a restoring force of F(x) = ax=bx^2, where a and b are constants.

when a certain spring is stretched by an amount x, it produces a restoring force of F(x) = ax=bx^2, where a and b are constants. How much work is done by an external force in stretching the spring by an amount D from its equilibrium length?