The warning is saying that, if all you do with the result from sprintf is to display it with disp, then you can display it with fprintf instead.
p1 = [5 6];
equacao_1=sprintf('The equation of the fitted function is: y= %f*x + (%f).',p1(1),p1(2));
disp(equacao_1)
fprintf('The equation of the fitted function is: y= %f*x + (%f).\n',p1(1),p1(2));
If you use equacao_1, the result from sprintf, somewhere else in your code to do something besides display it with disp, for example to set it to the title of a plot, then the warning will go away. Alternatively, you can right-click and disable the warning (on that line, in that file, or in all files), or set the warning to be disabled by editing your preferences (Preferences>Code Analyzer).
