I have to say this is wrong because you subtracted 3 and 3 is the variable term. So what you need to do is to add 9 to both side to get 3x by it self. Then divide by 3 and then x=6
the equation goes like this
y = ab^x
y = total cost
a = the intital amount (in your case the $1700)
b = the interest (since this is an exponetial growth it should be 1.013)
x = time
soo y = 1700(1.013)^x is the equation