Hey there! :)
In order to the find the missing side length of this triangle, we'll need to use the Pythagorean Theorem!
That equation is : a² + b² = c²
a = adjacent side
b = long side of rectangle
c = hypotenuse
We're already given the values of both a & c, so let's plug everything in!
a² + b² = c² → a = 10 , b = ? , c = 20
(10²) + b² = (20²)
Simplify.
100 + b² = 400
Subtract 100 from both sides.
b² = 400 - 100
Simplify.
b² = 300
Get the square root of b² & 300.
Simplify!
b = 17.3
So, the missing side length is the first answer choice : 17.3~Hope I helped!~