The surface area would be 126.
The equation for a rectangular prism is 2(width x length + height xlength +height x width)
This means our equation will be
2(5x3 +3x6 + 6x5), which can be broken down to
2(15+18+30), which gives us our final equation
2(63)
2 x 63 is 126, which means our surface are is 126 mi.
Hope this helps & good luck on your assignment <3!