Assuming that C is the hypotenuse...
The formula you want to use is:
a^2 + b^2 = c^2
We start by squaring 10, which is 100. 5^2 is 25. Now that we have the values, we can do the equation.
100+25=c^2 100+25=
Now, square root 
C= 11.180
You can solve this by setting up a proportion. 4/6 = x/9. Cross multiply to get 6x=36. Divide both sides by 6 to get x=6.
Hello,
(-5x-3)(-5x+
3) = (-5x)² - 3²