Subtraction of two fractions.

I'm learning math by reading books only and the book I'm currently studying is Smart Math by Marcia Learner in which I practice basic operation on fractions. I can't solve the two following problems. Let me show you how I did it but my answers don't match the answers the book presents.

1. $-7/8-(-1/3)=?$

$-7/8X3/3=21/24$ and $-1/3X8/8=8/24$ $-21/24+8/24=13/24$

My answer is $13/24$ but, according to the book the answer is $-1/2$

2. $12/13-(-3/7)=?$

$84/91-(-39/91)$ $84/91+39/91=123/91$

Book's answer is $105/14$

Please tell me where I've done the mistakes. I correctly did the other problems of similar kind but couldn't understand where I've done wrong.

I'm learning math by reading books only and the book I'm currently studying is Smart Math by Marcia Learner in which I practice basic operation on fractions. I can't solve the two following problems. Let me show you how I did it but my answers don't match the answers the book presents.

1. $-7/8-(-1/3)=?$

$-7/8X3/3=21/24$ and $-1/3X8/8=8/24$ $-21/24+8/24=13/24$

My answer is $13/24$ but, according to the book the answer is $-1/2$

2. $12/13-(-3/7)=?$

$84/91-(-39/91)$ $84/91+39/91=123/91$

Book's answer is $105/14$

Please tell me where I've done the mistakes. I correctly did the other problems of similar kind but couldn't understand where I've done wrong.