To solve for this, let's look at the information that we know...
26 students go on the trip
15 students pay $3 more for an additional class (x+3)
26-15=11
11students pay, x, the price of the trip.
So 15(x+3) represents the 15 students who pay for both the trip and the automotive class, and 11x represents the students who simply just pay for the cost of the trip.