A rectangle has four sides, two equal sides (length) and two shorter but equal sides (width or breadth). The perimeter is just the sum of all four sides which is twice the sum of length and width.
The formula for the perimeter is therefore 2(L+W). If L=24 and 2(L+W)=84in, we can find W. L+W=42in and W is 42-24=18in.
If we know L and W we can work out the area=LW=24*18sq in=432sq in. So the formula for area may also be used.