Hi can someone please help me with the following question:

i know and understand the definition of a metric space:

I can prove easily M1 and M2 but M3 is giving me problems. Can someone please help me.

M1: e(p,p)=0 by defintion.

e(p,q)>0 if p does not equal to q

M2: e(p,q)= (lpl+lql)=(lql+lpl)=e(q,p)

M3: really need help

Thanks

nerdo