The speed of a subway train is represented by the equation z=s^2+2s for all situations where 0≤s≤7, where z is the rate of speed in kilometers per hour and s is the time in seconds from the moment the train starts moving. In kilometers per hour, how much faster is the subway train moving after 7 seconds than it was moving after 4 seconds? Pick one.
A) 4 B) 9 C) 39 D) 48 E) 63
Given: z=s^2+2s for 0≤s≤7   z(4) = 4^2 + 2*4 = 24   z(7) = 7^2 + 2*7 = 63   Therefore z(7) - z(3) = 63 - 24= 
<gadget id="calculator">63 - 24</gadget>
<output>39</output>
39 km/hr   Option 39
<result>C</result>
The speed of a subway train is represented by the equation z=s^2+2s for all situations where 0≤s≤7, where z is the rate of speed in kilometers per hour and s is the time in seconds from the moment the train starts moving. In kilometers per hour, how much faster is the subway train moving after 7 seconds than it was moving after 4 seconds?