Question 647480
This is simply perimeter.

Since the garden is in a shape of a rectangle, you know that opposite sides or going to have the same length. Depending on how you turn your head,  35 meters could be the width while 21 meters could be the length. It doesn't matter.

{{{P=2l + 2w}}}
{{{P = 2*35 + 2 * 21}}}
{{{P = 70 + 42}}}
{{{P = 112}}}

So, she needs 112 meters (units are important!) of fencing material.