how do i find the dew point temperature if the specific humidity is 3.5g/kg?

Answer 1

change the specific humidity to relative humidity. That in itself is a task, see equation (3) in https://www.vaisala.com/sites/default/files/documents/Humidity_Conversion_Formulas_B210973EN-F.pdf.

Then, you can use charts (or the ln formula) to find dewpoint. Your text probably has those.