The height of the triangle is 48/b or 48 over b. That's all I can tell you with the information given.
If a line's slope is

, the slope of the perpendicular line is

in this case, the given line has a slope 4, which is 4/1, so the perpendicular line has a slope/gradient
Since sale is 40% so the price is 60%
61*(60%) = 36.6
the tax is 5%
so the price after tax is: 36.6 * (1 +5%) = $38.43
V= wlh so the answer is 56.25 or 56 1/4