I think you're calling m incorrectly. To set an equation, you need an == AND an =. The former defines the equation, the latter gives your equation a variable name.
x = sym('x')
eqn = m == (88140000/x^(39/50) + 42590)/(12700000000000000/x^(103/25) + 575700)
result = solve(eqn,x)
I'm not sure if this will return the answer you want, but it'll return a symbolic expression.