Lines a and b are parallel. The slope of line b is 13. What is the slope of line a?

Ok so we know that the lines A and B are parallel.

The term "parallel" implies that they will never meet each other, and to do so, the lines must have the same slope.

So the slope of line A is the same as line B (as for all parallel lines have the same slope)
So the slope of line A is also 13

Hope this helped