(12x + 5t)-(14t-6x)

12x + 5t - 14t -6x ( I get lost here when the -6x changes to +6.

Same as this question:
(5x + 2z)-(2x - 11z)

Can anyone explain this to me? Thank you

(12x + 5t)-(14t-6x)

12x + 6x - 14t + 5t
18x - 9t

Try the next one and we'll check it for you.