Answer: It has one solution. The solution is (x,y) = (-4,-3)
Add up the equations doing so straight down
x + -x = 0x = 0 so the x terms go away
2y + 2y = 4y
-10 + (-2) = -12
We end up with 4y = -12 so y = -3 after you divide both sides by 4. Use this y value to find the value of x
x+2y = -10
x + 2(-3) = -10
x - 6 = -10
x = -10+6
x = -4
The single solution is (x,y) = (-4,-3)
As a check, plug this solution into each equation to see if you get a true statement or not. Let's do so with the first equation
x+2y = -10
-4 + 2(-3) = -10
-4 - 6 = -10
-10 = -10 .... true
and then the second equation
-x+2y = -2
-(-4) + 2(-3) = -2
4 - 6 = -2
-2 = -2 .... true
both equations are true, so the solution is confirmed
I think the correct answer is 612
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.
Angle ABC=DEF then
Side AB=DF
Side BC=EF
and Side CA=FD
So if side ac=DF
side DF=6cm.