Since they are perpendicular the slopes of the two lines must be opposite (meaning they must have opposite signs such as "1" and "-1") as well as be reciprocals (meaning the fraction must be flipped, in example with this problem as the reciprocal of "-4/3" is "-3/4")
the slope of line L must be 3/4
The height is 5 becuase if the area is 35 that means the length is 7 so you would divide and get 5
It is the distributivity of the multiplication from the subtraction
8*(d-5)=8*d-8*5=8d-40