What is the area of this rectangle?

50 units²

Step-by-step explanation:

It's helpful, but not absolutely mandatory, to sketch the rectangle.  Doing this clearly positions the length and width of the rectangle.

The length of the line segment connecting A and B is the width of the rectangle, and that connection A and D is the length.

Use the Pythagorean Theorem to find the length and width:

length:  Moving from A to D, we see that x increases by 8 units and that y increases by 6 units.  Thus, the length of line segment AD is

|AD|² = 6² + 8² = 10², so |AD| = 10.

Similarly, the length of line segment AB is

|AB|² = 3² + 4² = 5², so that |AB| = 5.

The area of the rectangle is thus (length)(width) = (10)(5) = 50 units²