Answer:
 -2.33 m/s²
Step-by-step explanation:
The computation of the skateboarder’s acceleration is shown below;
Acceleration means the change in velocity per unit with respect to time. 
In the given case, the initial velocity is 7 m/s. 
As in the question it is mentioned that it comes to a stop, so the final velocity would be zero. 
And, The time elapsed is 3 seconds. 
Now the following equation should be used 
 a = (v,final - v,initial) ÷ t 
= (0 - 7)/3 
= -2.33 m/s²