It isn't really reasonable to want an answer within a half hour. Your question isn't clear, do you mean you want to get the speed of an object? You can calculate it with the Pythagorean theorem and the x and y velocities. Maybe this topic may help:

But what you replied with is math too, and actually it's basically the the same as what I referenced.

Velocity is a vector. Or basically it has a x and y value. The magnitude of a vector is it's length which can be calculated with the Pythagorean theorem:

Understand thanks your answer but in the c2 box2d physics why not maked defaultly magnitude and sqrmagnitude for a better physics? And in the unity why is? Magnitude is a often used members.

I do not like to code and math therefore I like c2 but I often feel that c2 expressions and built in members very few.