Model fitting with break point

I have a function: y=A(x/xc)^mc for x>xc y=A(x/xc)^mc for x<xc

c is the break point in the data which i don't know. mc is the slope for points above/below break point (depending on the condition). A is value of y at break point and x is value of x at break point i.e. c. How will I do it. x and y are known quantities.