The roots of an equation are the answers to the equation, where the graph of the equation intersects the x-axis. One can see that the points at which the graph intersect the x-axis are the following,
(-7, 0) and (1, 0), therefore the roots to the equation are the following;
It is proportional because the relationship between gallons of gas and distance is constant and linear. The rate is 32 miles per gallon. You can also call 32 the slope of this linear relationship.