B/13-3b/13=8/13 help please

-  = 
Since the two fractions have a common denominator already, we can just subtract the numerators.
b-3b=-2b or -3b+b=-2b
Either way you get the same answer :)
Our new fraction is 
Now we know that this must be equal to 
Again, since the denominators are already the same of course they are equal. 13=13
Now we have to set the numerators equal
-2b=8
Use inverse operations to get b alone
-2b=8
÷-2   ÷-2
b=-4
If you have any questions, feel free to ask. Hope this helped! :)

The answer is: a = about 63.59ft2 for 15