Let a number be "n"
n - 7 = 15
To do this first add 7 to both sides (what you do on one side you must do to the other). Since 7 is being subtracted, addition (the opposite of subtraction) will cancel it out (make it zero) from the left side and bring it over to the right side.
n - 7 + 7 = 15 + 7
n + 0 = 22
n = 22
7 less then 22 is 15
Hope this helped!
~Just a girl in love with Shawn Mendes