The important thing in this type of questions is to keep track of what you are doing at every point.
From t=0 to t=1s, there is uniform motion at v=24 m/s.
Therefore, it will move 24m in that second of time.
Afterwards, an acceleration comes in, so from t=1s to 11s there will be uniformly accelerated motion with acceleration -6m/s^2.
To account for this, you need to use Suvat's equation:
x(t)=x0+v0*t+1/2*a*t^2.
You should know what to plug in for each of the symbols in this equation:
x0 [initial position at t=1s] = 24m
v0 [initial velocity at t=1s] = 24 m/s
a [acceleration, switched on at t=1s] = -6 m/s^2
t [time from the start of the acceleration until the end, i.e. from t=1s to t=11s] = 10s
Plugging in those numbers in the equation will give you the position at t=11s.
