The average acceleration of the shuttle during its 7.5 minute run is 18m/s what velocity does it theoretically achieve at the end of that time?

Convert 7.5min to s. Hence, 7.5min = 450s
I am assuming the initial velocity is zero here and that 18 is the acceleration and not the velocity. 
Therefore you use this formula:
acceleration= (vfinal)/t [Note: velocity initial is not included as it is zero, so it would just cancel out]
 vfinal= a*t
          =18m/s²*450s
          = 8100m/s or 8.1*10³m/s if you correct to proper sigdigs.
Hope this helps!

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