Mary weighs 488 n and she walks down a flight of stairs to a level 4.1 m below her starting point. what is the change in mary's potential energy? answer in units of j.

Mary of course loses potential energy. The work done by Mary as she mounted the stairs is force * distance progressed. Her weight is the force she must overawed and the space moved is 4.1 m.
Potential energy of Mary is decreased so we put negative sign 
Change in energy = - m * g * h but mass = 488 / 9.8 
= - (488*9.8*4.1) / 9.8
For the reason that (Weight = mass * g) 
= - 19607.84 / 9.8
= - 2,000.8 Joule as she descends the stairs.