Since both equations are equal to y you can just make them equal to each other, like this 3x + 7 = x - 9. From there isolate the varible x on one side so that the solution will be on the other side of the equation. I'll do the steps too for you.
3x + 7 = x - 9
2x + 7 = -9
2x = -16
x = -8
Hope this helps :)
We have that F is proportional to m1. Hence, we have that F=m1*c when we change the first mass. We also have that F=m2*c when we only change the 2nd mass. Since F scales inversely with the square of the distance, we have that

. Combining all of the facts above, we get that F=c*

where c is a constant.
45.36km/hr.............. ...........
Ok so you can see 2 triangles that i highlighted, flip it so it is the same direction as you can see from the drawing
using the similar triangle equation
you get
136/64 = x/136
cross multiply and get
64x=18496
x=289
hope it help