For a star with a parallax angle of 1/2 of an at arcsecond, what will be its distance in parsec?

(1 parsec) is the distance at which an object has a parallax of 1 arcsecond. The distance is about 3.26 light years.

Another way to understand it is:  The distance from which the Earth's orbit appears 1 arcsecond across.

For a parallax angle of 1/2 arcsecond, the distance is 2 parsecs (about 6.52 light years).

1 arcsecond is 1/3600 of a degree, 0.00028 degree.  

a and b sometimes have a positive value