The perimeter of a rectangle is given by

p = 2(length + width)

You want to find the width when

p > 84 cm and length = 16 cm.

Substituting for p and for length,

2(16 cm + width) > 84 cm (the result of substituting the given information)

16 cm + width > 42 cm (divide both sides by 2)

width > 26 cm (subtract 16 cm from both sides)

Rectangle widths greater than 26 cm will make the perimeter greater than 84 cm for the given length.

p = 2(length + width)

You want to find the width when

p > 84 cm and length = 16 cm.

Substituting for p and for length,

2(16 cm + width) > 84 cm (the result of substituting the given information)

16 cm + width > 42 cm (divide both sides by 2)

width > 26 cm (subtract 16 cm from both sides)

Rectangle widths greater than 26 cm will make the perimeter greater than 84 cm for the given length.