The pythagorean theorem states that the sum of the squares of two legs of a right triangle is equivalent to the hypotenuse.
So:
35^2 + b^2 = 40^2
1225 + b^2 = 1600
b^2 = 375
b =
b = 19.3649
<u>The missing side's length rounded to the nearest tenth is Option C, 19.4</u>