Whenever you face the problem that deals with maxima or minima you should keep in mind that minima/maxima of a function is always a point where it's derivative is equal to zero.
To solve your problem we first need to find an equation of net benefits. Net benefits are expressed as a difference between total benefits and total cost. We can denote this function with B(y).
B(y)=b-c
B(y)=100y-18y²
Now that we have a net benefits function we need find it's derivate with respect to y.

Now we must find at which point this function is equal to zero.
0=100-36y
36y=100
y=2.8
Now that we know at which point our function reaches maxima we just plug that number back into our equation for net benefits and we get our answer.
B(2.8)=100(2.8)-18(2.8)²=138.88≈139.
One thing that always helps is to have your function graphed. It will give you a good insight into how your function behaves and allow you to identify minima/maxima points.
Answer:
S.I= 640
Step-by-step explanation:
I=prt
where p= $8,000
and r= 4%
and t= 2 years
$8,000×4%×2
8,000×4/100×2
80×4×2
=640
Therefore The S.I = 640.
Not sure if your trying to solve for x or v
Answer:
B. (-2,-4)
Explanation
Given equations:
y = 3x + 2
y = -2x - 8
Solving both equations will yield the values of x and y;
Solution:
y = 3x + 2 ----- (i)
y = -2x - 8 ------ (ii)
Using substitution method, input equation i, into ii
3x + 2 = -2x - 8
Collect like terms and solve;
3x + 2x = -8 -2
5x = -10
x = -2
Then put x = -2 into i, to find y
y = (-2 x 3) + 2
y = -6 + 2 = -4
So, the solution of the equation is B. (-2,-4)