Sin, cos, tan is the topic. how do you find the answer out for this ?

-- First, you look at the picture.  That's ALWAYS the first step
when you want to solve a problem and it has a picture with it.

-- You keep looking at the picture until you know:
. . . . What's the question ?
. . . . What's in the picture that shows what I have to find ?
. . . . What information is in the picture that I can use ?
DO NOT stop looking at the picture until you can answer THESE questions.

In this picture, it seems to me:
. . . . The question is:  Find the value of 'x'.
. . . . It's the length of the vertical side of the triangle.
. . . . The picture tells you ...
         . . . The triangle is a right triangle.
         . . . The triangle has a 40-degree angle in it.
         . . . The side adjacent to the 40-degree angle is 9 units long.
         . . . The 'x' that I have to find is the side opposite the 40-degree angle.

OK.  What's in your tool box ?  What have you learned in this class ?

Do you know anything about the side opposite an angle and the side
adjacent to the same angle, in a right triangle ?

You said the topic is the sin, cos, and tan functions.
Does one of these have to do with the sides opposite and adjacent
to the angle ?
Do you remember what the tangent of an angle is ?

The tangent of an angle in a right triangle is

           (the side opposite the angle)
divided by
           (the side adjacent to the angle).

Look at that !  In this triangle, the tangent of 40° is (x / 9) .

You can use a book or Google or your calculator
to find out the tangent of 40°.  It's a number.
Once you have it, you can write

                 (the number)  =  x / 9 .

Then you can multiply each side of that equation by 9,
and it'll say
                     x = (9) · (the number).

Then you'll know the length of 'x' in that triangle.
I'm pretty sure that's the problem you're supposed to solve,
and you'll have the answer right there in front of you.