See the attached image to see the alternate interior angles that are congruent (due to the parallel horizontal lines)
tan(angle) = opposite/adjacent
tan(31) = y/400
400*tan(31) = y
y = 400*tan(31)
y = 240.344247611024 <<-- use a calculator; make sure you are in degree mode
y = 240.3
Answer: 240.3
