Solution: 
We need to find 15-7 using two subtraction equation. 
First we split 15 and 7 as addition form. 
Split 15 = 10 + 5 
Because sum of 10 and 5 is 15 
Split 7 = 5 + 2 
Because sum of 5 and 2 is 7 
Now we substitute into original expression. 
(10 + 5) - (5 + 2) 
Now we open the parentheses 
10 + 5 - 5 - 2 
We can see two positive number and two negative number.
Now we move one positive and one negative number together 
10 - 5 + 5 - 2
Now we close in parentheses. 
(10 - 5) + (5 - 2)
Two subtraction equation: 
10-5 and 5-2