You can put this solution on YOUR website! When Trig. problems refer to "exact values" it means that you are to use one or more angles that are multiples of 30 or 45 (including 0) to find the value.
Since -225 is already a multiple of 45 (it's 45*(-5)), I don't really see any need for using sum or difference formulas. But since you were asked to do so, we will now look for a sum or a difference of angles that are multiples of 30 or 45 that result in -255. Once such combination that comes to mind: -225 = 135 - 360. (There are many other possible combinations that work.)
So tan(-225) = tan(135 - 360) and we can use the tan(A-B) formula to solve this.
Using 135 for A and 360 for B we get:
Since tan(135) = -1 and tan(360) = 0:
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