Answer
63$u$
Work Step by Step
The components of the $NH_3$ have the following atomic masses:
$H = 1u; N = 14u; O = 16u.$
So, the molecular mass of nitric acid will be the atomic mass of each element multiplied by the number of atoms of each.
MM $(NH_3)= (1\times1)+(1\times14)+(3\times16)=63u$