My fiancee asked this question and I am trying to figure it out but I am hitting a wall. I think that you would convert how much of a precious metal (gold or silver) that you could get with that much and then convert it to USD then but I am confused by the fact that the UK was on an unofficial gold standard at the time. Help would be appreciated.
What you're really asking about is the inflation from 1689 to today, and there's not a straight forward way to measure that because different goods and services inflate at different rates. You could calculate the value of one good, like gold or silver, but that doesn't tell you much about the purchasing power of 9 pounds in 1689. For that, you have to look at the price of goods, deed records, that sort of thing, and you'll get a more general idea of what you could have bought with 9 pounds in 1689.
But, if it helps, 9 pounds would have bought you 12 old sowes if you were living in North Carolina.
The way you want to do it, a straight valuation of that much silver in today's US$, 9 pounds of silver would be worth $2,145.45.
(14.58 troy ounces of silver to a pound. Current price of silver = $16.35 per ounce.)
However, as r/Tai15 explained, this won't really tell you much about the purchasing power of 9 pounds of silver in 1689.