Answer
The width of the screen is 18.43.
Step-by-step explanation
Use the Pythagorean Theorem (a^2+b^2=c^2) to find the height.
In a right triangle, a and b are legs. In this instance, a and b would be the height and width of the computer monitor. Let's say the height is a and the width is b (you're trying to find b). The hypotenuse of a right triangle is c. For the computer monitor, c is the diagonal.
So put in everything you know to find b; 12^2+b^2=22^2.
12^2 is 144 and 22^2 is 484. Now you have 144+b^2=484. When you simplify, you get b^2=340. When you simplify again, you find that b is about 18.43.