How do I find the reletive mass of 65^Cu? Don't give the answer, just tell me how to solve the problem?

Relative mass is just the relative abundance times the mass of the isotope.  

So for Cu65 you take the relative abundance 0.3083 and multiply it by the mass of Cu65 64.9277895 amu.  

By the way, you have the mass of Cu63 listed as the relative mass.  that's wrong, you multiply 62.929 times the relative abundance to get the relative mass.

The atomic mass found on the periodic table is the sum of the fractional abundance of each isotope times its respective mass.  Thus, for Cu the atomic mass is the fractional abundance of 63^Cu times its atomic mass plus the fractional abundance of 65^Cu times its atomic mass.  The equation would look something like:

Cu atomic mass = (63^Cu abundance*mass) + (65^Cu abundance * mass)

Then solve for 65^ mass.  Subtract (63^Cu abundance*mass) from both sides.  This gives:

Cu atomic mass - (63^Cu abundance*mass) = (65^Cu abundance*mass)

Divide both sides by 65^Cu abundance to get:

(Cu atomic mass - (63^Cu abundance*mass))/65^Cu abundance = 65^Cu mass