Increasing the price by $5 reduces demand by 20 units, so the slope of the curve is -4 units per dollar. This lets us write a demand equation as ...
q = -4(p -50) +184
q = -4p + 384
q = 4(96 -p)
The revenue is the product of price and demand:
r = pq = 4p(96 -p)
This is the equation of a quadratic curve that opens downward and has zeros at p=0 and p=96. The vertex (maximum) will be halfway between the zeros, at ...
p = (0+96)/2 = 48
A price of $48 per unit will yield a maximum total revenue.
The fastest way to do this is to convert both equations into slope-intercept form and graph it to find the solution point. If you wanted to do this algebraically, you might want to start out by getting rid of the fractions and using either substitution or elimination to find x and y.
