Actually, you cannot.
Any rectangle has four sides, two pairs of which have equal length. If we denote the length of one side as x (inches), then one more side has the same length, and two remaining sides have length y each.
Perimeter, the sum of all side's lengths, is clearly
`2x + 2y = 43.`
There are many (infinitely many) combinations of x and y. For any y between 0 and 43/2 = 21.5 we can find x,
`x = 21.5 - y.`
(or for any such x find y).
To find both x and y we need one more condition. For example, if area of a rectangle is given, or ratio of side's lengths, or diagonal length, or so. In such case we have a system of two equations of two variables which often have one solution.