A farmer makes a rectangular enclosure for his animals. He uses a wall for one side and a total of 72 metres of fencing for the other three sides.
The enclosure has width x metres and area A square metres.
Show that A = 72x - 21.

Remember that, for a rectangle of length L and width W, the area is:

A  =L*W

And the perimeter is:

P = 2*(L + W)

In this case, we know that:

W = x

Let's assume that one of the "length" sides is on the part where the farmer uses the wall.

Then the farmer has 72 m of fencing for the other "length" side and for the 2 wide sides, then:

72m = L + 2*x

isolating L we get:

L = (2x - 72m)

Then we can write the area of the rectangle as:

A = L*x = (2x - 72m)*x

A = 2*x^2 - 72m*x

(you wrote  A = 72x - 21, I assume that it is incorrect, as the area should be a quadratic equation of x)