The speed of the car increases from 26 mi/h to 56 mi/h in 5 s.

Use the formula v = u + at, where v is the final speed after time t, u is the initial speed and a is the constant acceleration.

Substituting the values for u, v and t gives

56 = 26 + a*(5/3600)

=> 56 - 26 = a*(5/3600)

=> 30*(3600/5) = a

=> a = 21600 mi/h^2

**The constant acceleration required is 21600 mi/h^2**