How many terms of the series 4,10,16, ....... must be taken such that the sum is equal to 602?

So far i have:

Sn=n/2(2a+(n-1)d)

602x2=n(2x4+(n-1)6)

1204=n(8+6n-6)

1204=8n+6n squared-6n

1204=6n squared+2n

602=3n squared+n

n+3n squared -602=0

I have got my self into a bit of a pickle with this now and am stuck any help would be much appreciated

cheers