I can't get this question.

Let f(x)= {-3x +7 if x<-1 and 10, if x>=-1}

Find the average rate of change in the interval -5<x<5.

I know the formula should be S(t1)-S(t2)/t1-t2, but i don't know what i'm doing wrong. I keep getting 12/10 as an answer.