GCSE Physics momentum question help

https://isaacphysics.org/s/jRz6rc (this is the question if you wanted to see it in detail)
I'm completely stuck with a physics question on Isaac Physics. I realise that you aren't allowed to give me the answer, but I would appreciate it if you could tell me what I'm doing wrong in my working and direct me on the right path.
The question is in the link (but if you can't click it or something I pasted it):
Two trolleys are at rest and in contact on a smooth, level surface. A coiled spring in one trolley is released so that they 'explode' apart. The lighter trolley moves off at 50cm/s. (picture on link) Find the minimum energy which was stored in the coiled spring before the release.

I got the speed of the other trolley which was 20 cm/s (no idea why but that doesn't matter lol)
My working for this was:
kinetic energy = 1/2 x mass x speed^2
= 1/2 x 7 x 0.7^2 (because it's 0.7 m/s if you convert)
= 1.715

Isaac Physics says this is wrong, I've tried it with all numbers of sig figs, and I tried not converting from cm/s to m/s and it still said I was wrong, so I would really appreciate some help.
(copy pasted - I think this was in the wrong forum before, sorry if you're seeing this twice)

Ugh ignore this, my laptop won't let me delete and I already have an answer but thanks anyway!

Was i right?