Suppose a line l has an equation ax+by+c==0, what is the slope of l if b=0 and a does not equal 0

The first week samantha learned to read braille, she could read 10 words per minute. in the second week she increased her speed by 20%. in the third week she increased her speed again by another 25%. how many words could she read per minute by the third week? hurry and tell me!