The area of a desk top is 8 ¾ square feet. if the length is 3 ½ft., find the width.

Area is calculated as length times width.
A = lw
Is the area and length are known, then you can plug in their values and isolate w to find the width.
I find it easiest to convert to decimals rather than using mixed numbers.
8 ¾ = 8.75 (area)
3 ½ = 3.5 (length)
Now let's plug in the values.
8.75 = 3.5w
To isolate w, just divide both sides by 3.5.
8.75 / 3.5 = 2.5
This means w = 2.5.
So the width is 2.5 feet.
Now convert the decimal back to a mixed number.
2.5 = 2 ½

So the answer to this question is that the width is 2.5, or 2 ½.

Hope this helps!