So let's name the numbers x and y.
we know that their sum, x+y is

and that that their difference, x-y is

so, from the last one we can find out x:
x-y=

x=

+y
we can substitute this into the first one:

+y+y =


+2y =

2y=

(7-2=5, but you have to "borrow" one because the fraction of the number from which you substract is smaller than of the other number)
so y=

then x will be
x=

+

x=