At 3:00 P.M., a bank robber is spotted driving north on I-15 at milepost 140. His speed is 134.0 mi/h. At 3:37 P.M., he is spotted at milepost 184 doing 103.0 mi/h. (Assume a straight highway). Assume north to be positive. What is the bank robbers displacement during this time interval?

Enter a positive value if the displacement is toward north and enter a negative value if the displacement is toward south.

My answer: -50.27 mi (I don't know what I'm doing wrong.)