e1:2*x - 2*y +  z -  w = 7;
e2: - x + 4*y - 5*z + 3*w = 1;
e3:x +  y -  z +  w = 4;
e4:-4*x + 2*y +  z - 2*w = 6;
linsolve([e1,e2,e3,e4],[x,y,z,w]);

solve(x^3-21*x+20=0);

plot2d([x^2-2*x+3],[x,-5,5]);
plot2d(cos(2*x),[x,-4,4],[color,green]);
plot2d(sin(1/x),[x,-0.1,0.1],grid2d);
plot3d(x^2+y^2,[x,-1,1],[y,-1,1],[palette, [gradient, red, orange, yellow, green]]);
plot3d(x^2-y^2 + 2*x*y^2+1,[x,-3,3],[y,-3,3],[palette, false]);