Which number you multiply by in order to change 213 to radiant measure?

We're guessing that you mean "radian" measure.

To change an angle in degrees to one in radians, multiply it by pi/180 .

That number is about 0.0175 . (rounded)

You could also divide degrees by 57.296 . (rounded)

213 degrees = 3.7176 radians. (rounded)