Hello!
You can use the Pythagorean Theorem
[tex] a^{2} + b^{2} = c^{2} [/tex]
c is the hypotenuse which is the side across the right angle
a and b are the other sides
Put in the values you know
[tex] a^{2} + 6^{2} = 10^{2} [/tex]
Square the numbers
[tex]a^{2} + 36 = 100[/tex]
Subtract 36 from both sides
[tex] a^{2} = 64
[/tex]
Take the square root of both sides
a = 8
The answer is 8
Hope this helps!