Solving for voltage but getting opposite sign than answer
Chu's comment is the right tip.
To answer what you're doing wrong in a more general sense:
You only have the direction of the current sources reversed compared to your calcuated (Voltage/Resistance) currents.

Starting with Eqn 1, v1/5 is current flowing from v1 to ground.  The 3.5A current source is also moving away from v1.  So, if you're going to list all the current sources coming from v1, they should be the same sign (+) and have them added together (v1/5 + 3.5 + (v1-v2)/4).

Similarly for Eqn 2 &amp; 3, you just have the sign/polarity of the current sources inconsistent with your V/R currents.  So, -3.5A in Eqn2, and -2A in Eqn3.

Your consistent reversal of the current sources would explain your consistency in getting negative versions of the right answer.