Answer:
x=

Explanation:
This triangle is a right triangle. We know this because of the little square in the corner of the triangle, that represents a right angle.
Therefore, we can use Pythagorean theorem.
a^2+b^2=c^2
a and b are the legs, and c is the the hypotenuse.
In this triangle, we know that 6 and x are the legs, because they form the right angle. We know 8 is the hypotenuse because it is opposite the right angle.
So, a is 6, b is x, and c is 8, and we can substitute them in.
6^2+x^2=8^2
36+x^2=64
Subtract 36 from both sides
x^2=28
Take the square root of both sides

x=

So, the first choice is correct