Please explain to me what formula do I need to answer this question. An architect presents a 3 inch wide by4 inch deep by 3 inch tall model of a new...

Please explain to me what formula do I need to answer this question.

An architect presents a 3 inch wide by4 inch deep by 3 inch tall model of a new central campus dorm. If the final building foundation is 126 feet wide. How deep (in feet) will the building be?