Hi GeoGebra comunity!! It's my first applet in this site, and I don't know if it would be realy succefull... Althought, I think it will be usefull for students who are now studing 2 variables functions. This applet gets you a contour line of the f(x,y) function, with hight z=k that you specify by moving the Slide point. This point is made in a way that lets you enter numbers in the interval (-a,a) (you can change this a number too). At the right, you have a tridimentional representation of the same function. To have a vision of all the contour lines of the interval (-a,a) you can activate the trace of the conic (use the "up arrow" button), and move the k Slide. The other buttons are made to desactivate the trace and clean all the screen. I also put a dinamic colour to the Contour Line in a way that put an orange colour to the higher line and a green colour to the low one. Thanks to use and watch, and luck!!