First, choose variables to represent the length and width of this rectangle. Why not use L and W? Then the area is L*W=84 sq. in., and this equation could be solved for either L or W. If for L, then L=84/W.
The Perimeter of the rectangle is P = 2L + 2W. Substitute the above expression for L into this expression for P. Doing so eliminates L from your equation. Now consider graphing your equation for P(W). Does your graph show a minimum value? If so, at which W value?