interest rate 6.5%, time 18 months, simple interest $828.75 what is the principal

Use the formula for simple interest:

i = p*r*t

Then    i = ($828.75)(0.065)(1.5)
              = $80.80

6.5% must be converted to a decimal fraction:  0.065, and 18 months to years, specifically 1.5 years.