We start with the 2020 cost: $2.91 per watt. The cost decreases by 4.5% each year, so each year the cost is multiplied by 1 − 0.045 = 0.955.
For t years after 2020 the cost follows exponential decay: multiply the initial cost by 0.955 raised to the t power.
Therefore:
C(t) = 2.91(0.955)t
where C(t) is in dollars per watt and t is the number of years after 2020.