Hello, soojin89!

I assume you made a sketch . . .

Two mirrors are placed at a 90° angle to each other.

A light ray strikes one mirror 0.650 m from the intersection of the mirrors

with an incident angle of 36.5°.

The ray then travels a distance $\displaystyle d$ before reflecting from the second mirror.

(a) What is the distance $\displaystyle d$? Code:

D
C * - - * - - -
| /
| /
0.65 | /d
|θ/
|/
B *
|\
|θ\
| \
| \
| A

The ray come in from $\displaystyle A$, strikes the first mirror at $\displaystyle B$

. . and reflects to point $\displaystyle D$ on the other mirror.

$\displaystyle BC = 0.65,\;\;\theta = 36.5^o$

In right triangle $\displaystyle BCD\!:\;\;\cos36.5^o \:=\:\frac{0.65}{d} \quad\Rightarrow\quad d \:=\:\frac{0.65}{\cos36.5^o} \;\approx\;0.8086$ m.