the formula for the perimeter of a rectangle is 2l+2w. which means twice the length and twice the width added to together.
if you mean this:
x=21+2x
minus x on both sides
0=21-x
add 21 on both sides
21=-x
x=-21
than you subsitute
2×-21=-42
-42+21= -21
The perimeter is than -21