You want to multiply the period by 2.00/1.99.
Since period is T = 2pi sqrt(L/g) and period is proportional to sqrt(L), then a ratio of 2.00/1.99 in period means a ratio of 2.00/1.99 in sqrt(L).
So the ratio of lengths is (2.00/1.99)^2. You weren't given the length, so all you can give is the ratio, which you could express as a percentage increase. For instance if the number is 1.05, that's a 5% increase.
