Looking at the point on the blue line for f(x) (0,1) and the point on the red line for g(x) (0,-9), the difference of the Y-coordinates is 10 ( from 1 to -9 is 10).
SO to have the red line move down 10 units, you would need to subtract 10 from the original equation.
The correct equation would be the last one g(x) = 5× - 10