Find the point of diminishing returns xy for the function Rx

Find the point of diminishing returns (x,y) for the function R(x)0.7x4.8x 5x where R(x) represents revenue (in thousands of dollars) and x represents the amount spent on advertising (in thousands of dollars). The point of diminishing returns occurs at approximately (Type an ordered pair. Do not round until the final answer. Then round to two decimal places as needed.)

Solution

R(x) = -0.7x³ + 4.8x² + 5x  
R\'(x) = -2.1x² + 9.6x + 5
R\"(x) = -4.2x + 9.6 = 0
x = 2.286

R(2.286) = 28.15

Inflection point: (2.286, 28.15)