Answer
$359K=86^{\circ}C$.
Work Step by Step
Find the rate at which the gasoline supplies heat.
$$Q_H/t=(3.2\times 10^7 J/L)(1L/17000m)(21.8m/s)=41035W$$
We are told that the rate of work output is 7000 W.
The minimum temperature for the hot reservoir is if the engine were a perfect Carnot engine. Calculate the efficiency of the engine from the given data and then find $T_H$.
$$e=1-\frac{T_L}{T_H}=\frac{W}{Q_H}=\frac{W/t}{Q_H/t}=\frac{7000W}{41035W}$$
$$T_H=\frac{T_L}{1-e}=\frac{(273+25)K}{1-(7000W)/(41035W)}=359K=86^{\circ}C$$