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, $\displaystyle a^2+b^2 \:=\:c^2$, we have: .$\displaystyle a^2 + 21^2 \:=\:29^2 $
. . $\displaystyle a^2 + 441 \:=\:841\quad\Rightarrow\quad a^2 \:=\:400\quad\Rightarrow\quad a = 20$
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?