How can i replace a matrix number with 0

A=[-1,1,1;0,-1,0;0,0,-1;1,0,0];

I chose A(1,3) and A(3,3) which is including 1 and -1

And how can i replace them zero

Answer 1

[Iain]

 A(1,3) = 0;
 A(3,3) = 0;