Your only option is to fill in the given points. That will give you two equations from which you can work out k and a.
Fill in (1,7) gives the easy formula 7 = k·a
Fill in (3,175) gives 175 = k·a·a·a
If you replace k·a in the second one with 7, you get:
175 = 7·a·a => a²=25 => a=5
Fron this and the first eqn follows k=7/5
