I am using Active Maths 1. I am doing exercise 15.3 no. 6 (ix) which is

to simplify 3^8 * -3^7

I said that this is 3^8 * (3^7*-1), so (3^8 *3^7) *-1, becomes 3^15 * -1 so answer is -3^15,

but the answer in the book is -3^1.

Can someone help me and explain how this can be worked out. I did a similar problem ex 15.2, no. 10 (ii) and I got the correct answer by this method.

Thanks!!