Hello!
You can use the Pythagorean Theorem
![a^(2) + b^(2) = c^(2)](https://img.qammunity.org/2019/formulas/mathematics/college/3byu32yniwf4k67ty0w3p5iut2e0wsjpqf.png)
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
![a^(2) + 6^(2) = 10^(2)](https://img.qammunity.org/2019/formulas/mathematics/college/bqefquilkcqcad66fdkkmngzywa33z7mus.png)
Square the numbers
![a^(2) + 36 = 100](https://img.qammunity.org/2019/formulas/mathematics/college/2bi8jzixqtj593l794zm9nacnw0owhwqoq.png)
Subtract 36 from both sides
![a^(2) = 64](https://img.qammunity.org/2019/formulas/mathematics/college/z93rapwvis9wmxaaiuplvgmen6x0qn4nnt.png)
Take the square root of both sides
a = 8
The answer is 8
Hope this helps!