y=mx+b is the form of a linear function. When x=0 y=15 so b=15 and therefore:
y=mx+15. To find m we need to find another pair of x-y values. Choose (2,12):
12=2m+15, 2m=3, m=-3/2, so y=-3x/2+15.
If this is a linear function all the other pairs should satisfy the equation.
x=4, y=-6+15=9; x=6, y=-9+15=6; x=8, y=-12+15=3; all these match the given data.