Given,
The function of yield is Y = f(x) = 345 + 135x - 15x^2.
a) There is 5 pounds of fertilizer used per acre.
So, x = 5,
Substituting the value of x in yield function then,

Hence, if 5 pounds of fertilizer is used per acre then it yeild 645 bushels of apple per acre.
b)The function of yiled is Y = f(x) = 345 + 135x - 15x^2
Differentiating the function with respect to x then,

The value f'(5) is,

The value of f'(5) is -15 bushels of apple per acre.
c)Here f(5) is more than the f'(5) . The increase the yeild less fertilizer must be used.
Hence, the fertilizer are used in part b are more.