A standard rectangular billboard sign has a perimeter of 124 ft. The length is 6 ft. more than three times the width. Find the dimensions of the sign.

All I know is that I've got a 3w+6 and 124. But the equation 3w+6=124 does not work.

The answer is length, 48; width, 14. But I'm not coming very close to those. Any help is greatly appreciated. Thanks!