basic algebra question

[pole92]

I have a practice problem that im not understanding how their getting plus from minus.
(7x+1) - (3x-8)
7x -3x =4x this part im getting
1+(-8) to me =-7
the book is saying 1+8=9
how is the - turning into +.

 

[tkhunny]

Distributive Property of Multiplication over Addition.

Just like

2(3x - 8) = 2(3x) - 2(8)

and

A(3x - 8) = A(3x) - A(8)

We see

-(3x - 8) = -1(3x - 8) = -1(3) - (-1)(8)

 

[pole92]

That makes sense. Thank you