Need a explaining on the equations

14. Suppose sales of the fuel-efficient four-cylinder Ford Fusion rose by 4 percent in the first quarter. Over the same period, the average price of regular gasoline rose from 118.9 cents per litre to 129.4 cents per litre. Using the midpoint method, calculate the cross-price elasticity of demand between the Ford Fusion and regular gasoline. According to your estimate of the cross-price elasticity, are the two goods complements or substitutes? Does your answer make sense