58cm converted into mm

With a diagram is most likely referring to drawing a picture to represent it, i.e. if you had 1/4, drawing a box, dividing it into fourths and filling in one of the fourths

With words is likely asking you to write out the fraction in words, i.e. "One Fourth" rather than 1/4.

I'm not quite sure, that wording is really cryptic, but those are my best guesses.