Perimeter of a rectangle is 42cm and its length is x centimeter. Find its width
length --- x
width ----- y
2x + 2y = 42
x + y = 21
y = 21-x
so the width is 21-x, where x is as defined
width ----- y
2x + 2y = 42
x + y = 21
y = 21-x
so the width is 21-x, where x is as defined