A rectangular field is 40 m long and 30 m wide…

Pythagoras theorem: in a right triangle with the right angle at B, $AC^2 = AB^2 + BC^2$. The converse: if $AC^2 = AB^2 + BC^2$, the angle at B is a right angle. To find a hypotenuse, add the squares of the legs. To find a leg, subtract the square of the known leg from the square of the hypotenuse.