The given formula for the perimeter of a rectangle is given by the equation as:
![p = 21+2w](https://img.qammunity.org/2019/formulas/mathematics/high-school/wjd06y09ppmne7o948ds9qiszzkd6i5qni.png)
we have to determine the equivalent equation, which is used to solve for the variable 'w'.
Since,
![p = 21+2w](https://img.qammunity.org/2019/formulas/mathematics/high-school/wjd06y09ppmne7o948ds9qiszzkd6i5qni.png)
Subtracting '21' from both the sides of the given equation, we get
![p-21=21+2w-21](https://img.qammunity.org/2019/formulas/mathematics/high-school/wdhawlbzv7e3qhsj9l9xv1867iz4c2rt2t.png)
![p-21=0+2w](https://img.qammunity.org/2019/formulas/mathematics/high-school/5x2rrrsh4b7850btrwgvmxips7wgg5lwlp.png)
![p-21=2w](https://img.qammunity.org/2019/formulas/mathematics/high-school/czyhw1131lm3mpw77my68owuuyg9eb9obk.png)
Dividing by '2' from both the sides of the equation, we get as
![(p-21)/(2)= (2w)/(2)](https://img.qammunity.org/2019/formulas/mathematics/high-school/p0ndhaanlaa9huct9e19dt9sjzbkm9j4a8.png)
is the required equation used to solve for the variable 'w'.