Finding a value in an Implicit Function

I have a Function below of which I am trying to find the independent variable (x) when I am given the dependent variable (W) of an equation. For my situation,W = 250 and I need to find x somehow with the function:

   function W = weight(x)
      a = 0.15;                  
      b = 0.05;                   
      K = 2800;                   
      L = sqrt(a^2+(b+x)^2);      
      Lo = sqrt(a^2+b^2);        
      dL = L-Lo;                  
      SINtheta = (b+x)/L;         
      W = 2*K*dL*SINtheta;        
   end

I cannot rearrange the equation for this question and I was told that maybe a Secant Method could work for this but I have not had much luck.