There is no exponent implied. Over the 6 years she receives 6*3000=18000.
To make it exponential we need to have y=x^6=18000. So x=18000^(1/6)=5.12 approx.
The general expression might be y=x^t where t=time in years. There are other equations using exponents that might apply; for example, investing an amount of money using compound interest at a certain percentage rate: y=x(1+r)^t where r=rate and x is the amount invested.