Are you in a course where you are expected to be able to do that? If so, do you know what a "linear function" is? Do you know any formulas for linear functions?
Now use the given information, "with x-intercept 6" to get an equation with m and b in it. Call this equation (1).
Do you know anything about inverse functions? If so, then you should know that if (-8, -3) lies on the graph of , then (-3, -8) lies on the graph of y = f(x). Use this to get another equation with m and b in it. call this equation (2).
Now solve equations (1) and (2) simultaneously for m and b.