I have a rectangular piece of paper (ABCD), with a width of 3 and a length of 4. I fold it diagonally so that vertex A touches vertex C. What is the length of the crease in the paper?

I have a drawing, containing all that I know so far (its not to scale). The red line is what I am supposed to find out. Also, I do not want to use any trigonometry functions, such as arctan, tan, sin etc.

Thanks.