Answer
The work done by friction is $-552.5~J$
Work Step by Step
The total work done by gravity $W_g$ and friction $W_f$ on the skateboarder is equal to the skateboarder's change in kinetic energy. We can find the work done by friction:
$W_g+W_f = \frac{1}{2}mv^2$
$W_f = \frac{1}{2}mv^2 - W_g$
$W_f = \frac{1}{2}mv^2 - mgh$
$W_f = \frac{1}{2}(65.0~kg)(9.00~m/s)^2 - (65.0~kg)(9.80~m/s^2)(5.00~m)$
$W_f = -552.5~J$
The work done by friction is $-552.5~J$
