The answer would be X =90
the x intercept is where the line crosses the x axis or where y=0
so set y=0 and solve for x
0=-2x-1
solve for x
add 2x to both sides
2x=-1
divide by 2
x=-1/2
the x intercept is at x=-1/2, the point is (-1/2,0)
I'm assuming you're trying to solve for r.
First divide a to both sides
Now you have

= a - r
Subtract a to both sides.
And then divide by -1 to both sides.
Now your equation is -

+ a = r