This is an example of why you should have paid attention in school. The diagonal is the hypotenuse common to the two identical right triangles created when the screen rectangle is bisected through opposing corners. The Pythagorean Theorem tells us that the hypotenuse (which we know to be 42") is equal to the square root of the sum of the squares of the other two sides of the triangle. Or, put more simply, a-squared plus b-squared equals c-squared. Further, since we know the screen aspect ratio, we know the mathmetical relationship of one side to the other (the height is 9 times 1/16 of the width). As Holmes would say, the rest, my dear Watson, is elementary [algebra]!
Anyway, the short answer is that the width is 36.6" and the height is 20.6", give or take...