So we want to know how much work does Brianna do while climbing the stairs to a height h=8m while she weighs F=425N and in t=6s. So work W is defined as Force F times distance h or: W=F*h. Since Brianna has to use the force that is equal to her own weight during her climbing the calculation is this: W=F*h=425N * 8m=3400J. The work she does is W=3400 J.
