A field has a length of 12 m and a diagonal of 13 m what is the width?

Do Pythagorean Theorem because a rectangle divided in diagonals form right triangles

A^2+b^2=C^2

Plug in the correct numbers and solve

A and B can be switched around but C is always the longest diagonal side which is called the hypotenuse not the Adjacent sides that form a right angle

So with correct solving you should get 5.