How do you use Pythagorean theorem when you only know one side of the triangle?

Since the Pythagorean theorem has 3 variables (the 3 sides), and you only know one variable, you cannot use it to solve for the other 2 variables.

Do you know any other information (like angles)? that can be used to solve the triangle (you would not be using the Pythagorean theorem, though).

Did they mark the 2 legs with little 'tick marks' denoting that they are equal?

Then you could use the Pythagorean theorem because then you know 2 varaibles, and you can solve for the third variable.