Hello, xsriel!

This doesn't require any fancy math . . .

A right triangle has a side of 21 inches and a hypotenuse of 29 inches.

A second triangle is similar to the first and has a hypotenuse of 87 inches.

What is the length of the shortest side of the second triangle?

Using Pythagorus, , we have: .

. .

The first right triangle has side of 20, 21, 29.

It looks like this: Code:

*
29 * *
* * 20
* *
* * * * *
21

The second right triangle is similar to the first

. . and has a hypotenuse of 87.

It looks like this: Code:

*
* *
* *
87 * *
* *
* *
* *
* *
* * * * * * * * *

The hypotenuse is three times as large.

. . So the *entire triangle* is three times as large.

Got it?