You probably missed that when the numerator and denominator are both decreased by 2, the new fraction is now equal to 1/2.
The numerator of the fraction is "n" and the denominator of the fractions is "d". Initially the numerator and denominator are in the ratio 3 to 5. In equation form we can state this as:

When both numerator and denominator are decreased by 2, the new numerator will be n-2 and denominator will be d-2. These numerator and denominators are in ratio 1 to 2. In equation form we can write this as:

This is our second equation. Thus the two equation which can be used to solve the problem are:
5n = 3d
and
2n - 4 = d - 2