It's 5 less than -2. Negative numbers are less than 0, so to get smaller the number has to get bigger. -7 is smaller than -2, because it is further away from 0. So -2-5 = -7.

Why does -2-5=-7 ?

because if you go down 2 steps and then you go down another 5 steps then you will have gone DOWN 7 steps altogether :)