The sum of two numbers is 3, their difference is -15. find the numbers.

 Let, the numbers = x, y
So, x + y = 3
x - y = -15
x = -15 + y

Now, substitute the value of x in 1st equation, 
-15 + y + y = 3
2y = 3 + 15
y = 18/2
y = 9

Now, substitute the value of y in equation 2nd, 
x = -15 + 9
x = -6

In short, Your Numbers are 9 and -6

Hope this helps!