Answer: 13.
Explanation:
We know that,
When two lines are parallel then they have the same slope.
For example : If line m and n are parallel and line m has slope -1 , then the slope of line n would also be -1.
We are given that ,
Lines a and b are parallel.
⇒ They have same slope.
The slope of line b is 13.
⇒ The slope of line a would also be 13.
Hence, the the slope of line a = 13.