Question: about maple fitting

Hello,

How can I fit my equation with the experimental data given below? 

My equation is:  y = A*y^3+B*x ,  where x is the independent variable,  and y is the dependent variable. 
Also how can I determine A and B constants? 
Thanks

Bengu

restart;
Xvals := [.53993447, .5599647, .57995479, .59995566, .61996118, .63994512, .65994136, .6799731, .69996782, .71997949, .73997422, .75995044, .77994976, .7999707, .81995146, .83995244, .85996729, .87994951, .8999458];
Yvals := [-.79625455, -.75585259, -.67800183, -.47955884, -.25493698, -0.65747361e-1, .10114507, .26994542, .41484068, .50621122, .60363251, .65510417, .7251356, .75804148, .76002419, .8030069, .82774732, .8429559, .8692888];
with(Statistics);
points := ScatterPlot(Xvals, Yvals);
PLOT(CURVES(Matrix(19, 2, {(1, 1) = .53993447, (1, 2) = 

   -.79625455, (2, 1) = .5599647, (2, 2) = -.75585259, (3, 1) = 

   .57995479, (3, 2) = -.67800183, (4, 1) = .59995566, (4, 2) = 

   -.47955884, (5, 1) = .61996118, (5, 2) = -.25493698, (6, 1) 

   = .63994512, (6, 2) = -0.65747361e-1, (7, 1) = .65994136, 

   (7, 2) = .10114507, (8, 1) = .6799731, (8, 2) = .26994542, 

   (9, 1) = .69996782, (9, 2) = .41484068, (10, 1) = .71997949, 

   (10, 2) = .50621122, (11, 1) = .73997422, (11, 2) = 

   .60363251, (12, 1) = .75995044, (12, 2) = .65510417, (13, 1) 

   = .77994976, (13, 2) = .7251356, (14, 1) = .7999707, (14, 2) 

   = .75804148, (15, 1) = .81995146, (15, 2) = .76002419, (16, 

   1) = .83995244, (16, 2) = .8030069, (17, 1) = .85996729, 

   (17, 2) = .82774732, (18, 1) = .87994951, (18, 2) = 

   .8429559, (19, 1) = .8999458, (19, 2) = .8692888}, datatype 

   = float[8]), COLOUR(RGB, 0., 0.054901961, 0.47058824), 

  STYLE(POINT)), AXESLABELS("", ""), VIEW(DEFAULT, DEFAULT))
with(plots);
display(points);

Statistics:-Fit(a*y^3+b*x, Xvals, Yvals, y);
                              3                             
   HFloat(2.996866026020011) y  - HFloat(0.9272751314432008)
example("?");