An object moving with constant acceleration has a velocity v = 55.2m/s when it is at x = 6.9m and a v = 82.4m/s when it is at x = 12.4m. What is its acceleration?

Now this problem qould be simple if time were given but it isn't so I'm gonna need a little bit of help please.

What I did with this one was I plugged the numbers into the following equation:

Vf^2 = Vi^2 +2a(xf - xi)

which gave me,

(82.4)^2 = (55.2)^2 + 2a(12.4 - 6.9)

6789.76 - 3047.04 = 2a(5.5)

a = 340.25m/s^2

whish is needless to say, well....impossible. Is there another way to attack this problem or maybe if someone could please point out my mistake?