Take into account that the average power is given by:
[tex]P=\frac{F\cdot d}{t}[/tex]where F is the force, d is the distance and the t the time.
Then, it is necessary to calculate F. Use the following formula:
[tex]F=m\cdot a[/tex]m: mass of the car = 2000 kg
a: acceleration = ?
Calculate acceleration as follow:
[tex]a=\frac{v-v_o}{t}=\frac{\frac{0m}{s}-\frac{30m}{s}}{25s}=1.2\frac{m}{s^2}[/tex]where you have considered that the final velocity is 0m/s because the car stops.
Then, the force is:
[tex]F=(2000kg)(1.2\frac{m}{s^2})=2400N[/tex]Now, consider that it is necessary to calculate distance d in order to able to calculate P.
Calculate distance d as follow:
[tex]d=\frac{v^2_o}{2a}=\frac{(30\frac{m}{s})^2}{2(1.2\frac{m}{s^2})}=375m[/tex]Then, by replacing d, F and t = 25 s, you obtain for the power:
[tex]P=\frac{2400N\cdot375m}{25s}=36000W=36kW[/tex]Hence, the average power produced by the brakes was 36kW
