This cannot be solved. It needs 2 equations to solve for 2 unknowns.
If y equals 5x-15 and x equals 1 then the value of y comes out to be -10. So y equals 5x-15 is -10.
-10
y=-10 sin 5x sin 5x=y/-10 x=asin(y/-10)/5
x=-2, y=2
To work out y=-5x-10 you can plot a graph.
8
15
-5x=3+y x=-1/5y-3/5
Add: 5x - y + x + y + 8 + 10 ie 6x = 18 so x = 3 and y = 7
No solution
15
15