3x + 2 - 1.7 = 0.6x + 31.4

1.3x + 2 = 0.6x + 31.4

If you subtracted 1.7 from 3x in the first equation above to obtain 1.3x in the second equation, that's wrong.

3x and 1.7 __are not__ like-terms because one of them contains x and the other does not.

However, 2 and -1.7 __are__ like-terms (they're both constants), so you can combine them to get 0.3