Hey there :)
Let us first translate words into mathematics!
Sum of 2 numbers is 52
x + y = 52
Difference of 2 numbers is 9
x - y = 9
(eq.1) x + y = 52
(eq.2) x - y = 9
Add both equations : x + y = 52
+ x - y = 9
2x = 61
x = 30.5
We now have the value of x, substitute into eithe formulae to find y
x - y = 9 , 30.5 - y = 9 , 30.5 - 9 = y , 21.5 = y
x + y = 52 , 30.5 + y = 52 , 52 - 30.5 = y , 21.5 = y
x = 30.5
y = 21.5