Well all i did was guess a random number from 1-10 so I could see what numbers make the equation bigger than -18 and the fist thing I guessed was 5 so this is what I did
8-6*5=-22
And them I said let me see if any multiples equal -18 so I went to for and did this
8-6*4=-16
That's when I guessed the answer if i looked at the sign correctly!
Not sure te rate is r for decay, it is A=P(1-r)^t r=rate P=present amount dunno what b is but I guess A=28000(1-0.13)^t A=28000(0.87)^t I would guess the decay factor would be 0.87
