Answer
It will take Lars 13.0 seconds to climb three flights of stairs.
Work Step by Step
We can find the total amount of work that Lars must do:
$Work = mgh = (82.4~kg)(9.80~m/s^2)(12.0~m) = 9690~J$
We can find the time it takes to do this work:
$t = \frac{Energy}{Power} = \frac{9690~J}{746~W} = 13.0~s$
It will take Lars 13.0 seconds to climb three flights of stairs.
