The boat uses 4 gallons to travel 6 miles. Traveling 4 times as far is expected to require 4 times the gas.
4 gallons = 6 miles
Multiply by 4.
16 gallons = 24 miles
⇒ 16 gallons
The independent variable is is the price per bag.
Both have a right angle, and both have a 61* angle, meaning the third angle has to be 29* on both (all angles in a triangle have to add up to 180 so you subtract the 90 and the 61 = 29)