Answer
Sodium has a positive charge. Boron has a negative charge because it forms 4 bonds instead of 3.
Work Step by Step
Formal Charge (FC) = [number of valence electrons] - [number of non-bonding electrons] - 1/2 [shared electrons]
FC (Sodium) = 1-0-0 = +1
FC (boron) = 3-0-4 = -1