Having trouble with this problem and never been good at the slope.

First we need to find the base fee by setting up a systems of equation.
47.50 = 150x + b
70 = 300x + b
In this, x is number of miles driven and b is the base fee. Isolate x in one of the equations and solve by substitution.
70 = 300x + b > Subtracy b from both sides and then divide by 300.
x = (70-b)/300 > Now plug this into the other equation and solve for b.
47.50 = 150((70-b)/300) + b > Simplify and then isolate b.
47.50 = ((70-b)/2) + b
47.50 = 35 - b/2 + b
12.50 = .5b
b = 25
Now plug b into one of the equations above to get cost per mile.
70 = 300x + 25
45 = 300x
x = .15
Now use these values for your equation.
a.) f(x) = .15x + 25
b.) D
c.) For this just plug 310 into the equation you made.
f(310) = .15(310) + 25
f(310) = $71.5
Hope this helps!