It can be easily assumed that most information are already given in the question. Based on that information, the answer can be deduced.
rt = [(3/4)r] * (t + 12)
4rt = 3r (t + 12)
4rt = 3rt + 36r
4rt - 3rt = 36r
rt = 36r
t = 36
From the above deduction, we can conclude that the time taken by Carmen to ride to the market is 36 minutes
Then
Time taken by Carmen to ride home = 36 + 12 = 48 minutes