V = lwh
15(10.5)(4.5) = 708.75
In order for two lines to be perpendicular, their slopes must multiply together to equal to negative one
So we do negative one divided by 2/5 to get the slope of the red line
Which is -5/2
To check, we do 2/5 multiplied by -5/2 which equals to -1 so our answer is correct
Complete Question
The complete question is shown on the first uploaded image
Answer:
The point of diminishing returns (x , y ) is (11, 21462)
Step-by-step explanation:
From the question we are told that
The function is 
Here R(x) represents revenue (in thousands of dollars) and x represents the amount spent on advertising (in thousands of dollars).
Now differentiating R(x) we have

Finding the second derivative of R(x)

at inflection point 
So 
=> 
substituting value of x into R(x)


Now the point of diminishing returns (x , y ) i.e (x , R(x) ) is
(11, 21462)