When you formed the equation:
you assumed the direction of normal for plane P as "+x" and direcion of normal for plane E as "+z". Now, the these two planes divide the space in four parts. These four parts are:(10-6m)-8=11m
(1)Points lying above E and right of P
(2)Points lying above E and left of P
(3)Points lying below E and right of P
(4)Points lying below E and left of P
(Imagine a horizontal x-axis and vertical z-axis to understand the above four cases)
Now, you equation covers only the parts (1) and (4) (why?????)
When you consider the parts (2) and (3), you get m=-2/5.
If you can't work out yet, ask for details.