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The car starts at rest, so its velocity is 0. Then in 5 seconds, it accelerates to 25 m/s. So, if it has uniform acceleration, the acceleration is just the change in velocity over the change in time:
a = Δv/Δt, for uniform acceleration
a = (25 - 0)/5 = 5 m/s2
The formula for velocity is:
v(t) = v(0) + a*t
at v(0), the velocity is 25 m/s. Let's plug and
v(t) = 50 m/s = 25 m/s + (5 m/s2)*t
25 m/s = (5 m/s2)*t
t = 5 s
So, it will take another 5 seconds to reach 50 m/s.
This makes sense since it took 5 seconds to get to 25 m/s, so it'll take another 5 seconds to double that since it has uniform acceleration.