First you can factorise the equation by finding factors of 36 that add together to make 12. These factors are 6 and 6, and because the end number is positive we know that both signs are the same, and it's -12 so both are negative. This leaves us with
(z - 6)(z - 6) = 0
Because one of the brackets have to equal 0, the solutions are 6 and 6, which is answer B.
I hope this helps!
