we have the linear equation
C(x)=46x+88,000
This is a linear equation in slope-intercept form
where
the slope or average rate is m=$46 per college textbook
and
the initial value is b=$88,000
therefore
Part A
the fixed cost is the same that the initial value (value of C(x) when the value of x=0)
so
Fixed cost=$88,000
Part B
the marginal cost per book is the same that the slope
so
the marginal cost per book is $46 per college textbook
Part C
For x=800
C(x)=46(800)+88,000=$124,800
For x=31,000
C(x)=46(31,000)+88,000=$1,514,000
Part D
To find out the average cost you need to average total costs over the number of units produce
so
For x=800 books
average cost=124,800/800=$156
For x=31,000 books
average cost=1,514,000/31,000=$48.84