Answer:
When subtracting expressions, we are applying the distributive property, where a multiple of an expression in brackets is applied to to each term.
e.g.
a(x + y) = ax + ay
In the case of subtracting an expression, we're doing the same thing, but in the example above, "a" would be equal to -1.
(a + b) - (c + d) ≡ (a + b) + -1(c + d)
And so the negative one is distributed across the terms within the brackets.
