Find two numbers, if

Their sum is 3/5 and their difference is 7 1/5

Let the numbers be represented by x and y.

... x + y = 0.6

... x - y = 7.2

Add these equations to get

... 2x = 7.8

... x = 3.9

Then you can find y from

... y = 0.6 - 3.9 = - 3.3

The two numbers are - 3.3 and 3.9.