Originally Posted by KARH4 My homework problem reads: a truck slows down uniformly with acceleration of 5.1m/s2 for 4.20s, making skid marks 58.7m long the end of a tree. With what speed does the truck then strike the tree?
I know that vv0= at which would give me 21.42m/s 
yes, this is correct
And I also know v2v02= 2ad which would give me 598.74m/s.

yes
I tried taking the square root of a positive 598.74 thinking it may would give me the 21.42 answer but they don't seem to come out even. I've just hit a road block with this one. Thanks!

you could rewrite that equation as v_0^2 v^2 = 598.74 but when you take the square root of both sides, sqrt {v_0^2 v^2} is not the same as v  v_0, so you shouldn't expect to get the same result. Instead, try using one of the other kinematic equations, like d = v_0t + 1/2at^2, to solve for v_0 directly, then use your first equation to solve for v.
*I do realize the problem does contradict itself in that it reads 'acceleration' and then has a negative number. I think it was just a typo in the problem.

there is no contradiction, the minus sign implies that the car is slowing down, not speeding up.