Hey there!! 
Total length of the fabric bought = 3 1/4 yards 
First, let's convert this mixed fraction to improper fraction 
3 1/4 = 13/4 
The total cost for 13/4 yards = $13 
Find cost per yard
Let's take the cost per yard as ' x ' 
Then, 13/4 yards = 13x / 4 
Let's get this into an equation - 
13x/4 = 13 
Multiply by 4 on both sides 
13x = 52 
Divide by 13 on both sides 
x = 52 / 13 
x = 4 
The cost per yard = $4 
Hope my answer helps!