18x3=8(x+16)+10x Can someone please help me

Find the average rate of change for the given function drom x=1 to x=2

Martha is training for a duathlon, which includes biking and running. she knows that yesterday she covered a total distance of over 55.5 miles in more than than 4.5 hours of training. martha runs at a speed of 6 mph and bikes at a rate of 15.5 mph.

Pleas its really urgent and i would really appreciate it! what is the value of x? enter your answer in the box. x =