since 43.2 months is the mean, and the standard deviation is 7.7 months
that means if we take the z score:
now that we know how many standard deviations you are away from the mean: -0.9351
using the zscore table you can find the proportion, or you can use the calculator if you have one that can do that.
you get 0.1711 which in percentages is 17.11%