Answer:
The width of the shelf is 6 inches.
Explanation:
The leg (brace) attached to the wall is 4.5 inches.
The length of the hypotenuse = 7.5 inches.
Also, the Length of leg holding the shelf = Width of the shelf
Now, as the brace is in the shape of right triangle.
In a Right Triangle, by PYTHAGORAS THEOREM:

Putting here the values of hypotenuse and base, we get:

So, the length of the leg = 6 inches
Hence, the width of the shelf is 6 inches.