Right off the bat we know that the y-intercept is going to be 5 because of the second co-ordinate.
Now for the slope, because the y co-ordinate doesn't change, it has no slope so the equation is simply: y=5
Hope this helps :)
It be 81.
You have to take half of -18 which is -9, and square that, which is 81.
Y = mx + c
y = 2x + c
When x = 5 and y = 0,
0 = 2(5) + c
c = -10
y = 2x - 10