What is the minimum perimeter of a rectangle that has an area of 84 square inches?

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?