Astronomy Home Work how do i solve for the aphellion distance when i only have the perihelion distance.?

Then, you must either have the speed at that distance, or the major axis length, or the eccentricity of the orbit. 

Here's some formulas you can use, based on the information you might have: 
https://en.wikipedia.org/wiki/Elliptic_orbit

You can't unless you also know the semimajor axis or the eccentricity.

Do your own homework.

You should have something else, like the orbital period.