Question: Simplify the expression

I want to write the equation of the plane which equidistant the two the planes P and Q. This is my code

restart:

with(geom3d):

plane(P,2*x+3*y-6*z+1=0,[x,y,z]):

plane(Q,x -4*y-8*z+1=0,[x,y,z]):

point(M,x,y,z):

E:=distance(M,P)- distance(M,Q);

simplify(%);

but, i can simplify(E). Please help me.

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