That would depend on which of 3 and 5 that you consider the length. If we see this as a standard business card the length would be 3 in as it is 5 in wide as you look at it.
w/15=5/3 multiply both sides by 15
w=75/3 in
w=25 in
...
if you need it the other way around...
w/15=3/5 multiply both sides by 15
w=45/5 in
w=9 in