We are given the relationship:
a. It's required to find a relationship where r is a function of t. To do that, we need to solve the equation for r.
Subtract 4t:
Divide by 7:
b. We use the function found in part a and evaluate it for t=-7:
Thus, f(-7) = 6
c. Solve f(t) = 18
Again, we use the function from part a and solve the equation:
Multiplying by 7:
Subtract 14 and then divide by -4:
t = -28