When I get a question like this I like to transform it into a positive angle as I think it's easier to work with. If you add 360° on to the angle, the trig values stay exactly the same.
tan(-945)° = tan(-585)° = tan(-225)° and so on...
keep doing this until you get a positive value, in our case we get tan(135)°
This is in the second quadrant of a unit circle diagram so we know tan must be negative and make an angle of 45° to the x-axis so tan(135)°=-tan(45)°
tan(45)° = 1 therefore combining all this gives tan(-945)° = -1.
(I know there's a lot to take in there, if you have any questions, feel free to ask.)