You need to include numbers of dollars. You did include the rates.
Phyllis invested A dollars, a portion earning simple interest rate 4% and the rest earning 7% (is this 7% simple or compound?). After one year the total interest earned was $C. how much did she invest at each rate?
Let's assume both interest rates are simple interest rates. Phyllis ended up with C dollars. I assume C dollars is greater then A dollars. So your equations are:
A = D + E where D dollars was invested at 4% and E dollars at 7%, and
C = D(1.04) + E(1.07) since D dollars was invested at 4% and E dollars was invested at 7% and after one year you end up with (dollars)(1+interest rate) dollars.
Now you have two equations in two unknowns and so it's possible to find a solution. But you need to provide the numbers A and C.