I am trying to solve below question from Coursera Intro to Calculus ([link](https://www.coursera.org/learn/introduction-to-calculus/quiz/kVlbQ/the-theorem-of-pythagoras-and-properties-of-the-square-root-of-2))

> 
> A right-angled triangle has shorter side lengths exactly $a^2-b^2$ and
>  $2ab$ units respectively, where $a$ and $b$ are positive real numbers
>  such that $a$ is greater than $b$. Find an exact expression for the
>  length of the hypotenuse (in appropriate units).
> 
> 
> 

Below are the choices

$(a - b)^2$

$\sqrt(a^4 + 4a^2b^2 -b^4)$

$a^2 + b^2$

$\sqrt(a^2 + 2ab -b^2)$

$(a + b)^2$

When I attempt to work out the solution (and I even got a 2nd pair of eyes to look at it, but he arrived at the same conclusion), I get this:

$(a^2 - b^2)^2 + (2ab)^2 = x^2$

$a^4 -2a^2b^2 + b^4 + (2ab)^2 = x^2$

$a^4 -2a^2b^2 + b^4 + 4a^2b^2 = x^2$

$a^4 + 2a^2b^2 + b^4 = x^2$

$\sqrt(a^4 + 2a^2b^2 + b^4) = x$

Please help! How to get the correct solution?