What is your problems..?
It's very easy
13 + z = 2
ask it in another way
how to let the 13 becomes 2?
13 - [ ? ] = 2
you will get 11
but 13 + 11 will NOT equal 2
it should be MINUS
13 - 11 = 2
→13 + (-11) = 2
z = -11
Do the remaining question yourself.