#### Answer

$y = 2 \sqrt {21}$
$x = 10$

#### Work Step by Step

The altitude of a right triangle is the geometric mean of the lengths of the two hypotenuse segments. Let's set up that proportion:
$\frac{a}{x} = \frac{x}{b}$, where $a$ and $b$ are the lengths of the two hypotenuse segments and $y$ is the length of the altitude.
Let's plug in our numbers:
$\frac{21}{y} = \frac{y}{4}$
Use the cross products property to get rid of the fractions:
$y^2 = 84$
Rewrite $84$ as the product of a perfect square and another factor:
$y^2 = 4 • 21$
Take the positive square root of each factor to solve for $y$:
$y = 2 \sqrt {21}$
To find $x$, we know that each leg of the triangle is the geometric mean of the hypotenuse and the hypotenuse that is adjacent to that leg:
$\frac{a}{x} = \frac{x}{b}$, where $a$ and $b$ are the length of the hypotenuse and the length of the segment of the hypotenuse closest to the leg, and $x$ is the length of the leg.
$\frac{21 + 4}{x} = \frac{x}{4}$
Use the cross products property to get rid of the fractions:
$x^2 = 25 • 4$
Multiply to simplify:
$x^2 = 100$
Take the positive square root to solve for $x$:
$x = 10$