Matter, Antimatter and Einstein’s equation

It is my understanding that when antimatter comes in contact with matter that there is nothing left but pure energy. And since energy and matter are interchangeable (E=Mc2) doesn’t that mean you are actually left with all matter and no antimatter...

Jaz posted this question from electrostatics

“I’ve tried so many times, but I’m unable to arrive at the correct equation; See attachment for question” Question   Jaz posted this question E=k(2λ/r) due to positive wire E’=k(2λ/(D-r)) due to negative wire The r...