First, convert the speed to m/s to ease the calculations:
88 km/h is 88000m per 3600 seconds, so we need to divide that 88000 by 3600 to get it in m/s.
88000 / 3600 = 24.44 m/s
Now, we have the initial velocity of the car, and the final velocity is 0, and we also have the distance it slows down in (the length of the crumple zone).
Therefore we can calculate the acceleration:
u = 24.44
v = 0
s = 0.75
a = ?
v^2 = u^2 + 2as
0^2 = 24.44^2 + 2*a*0.75
0 = 597.5 + 1.5a
-1.5a = 597.5
a = -398.4 m/s^2
To two significant figures, a = -400 m/s^2
To put it in perspective, it's a little over 40g.