What are these values, and what is your hypothesis?
Are the values discrete or continuous. Counts or frequencies?
Are the two samples paired? (e.g. x is a single value after treatment to samples matched to those in y, but with poor survival)
Do you know the expected distribution of the values (e.g. can you assume normal distribution)
If the difference between x an y is great, you may just state the values and compare the samples (rather than do hypothesis testing for the underlying populations).
Alternatively, if you are testing whether the value x could come from the distribution y, you might employ the t-statistic. Calculate the mean, standard deviation and standard error of y using all values. The significance of your observation x from the mean of y is:
t-statistic = (valueX - meanY) / standard_error_Y= t stat.
Use the Excel TDIST function (DF, t stat, 1 (for one tail)) to get your p-value
Alternatively, use a single sample t-test in R : t.test(y,mu=x).
For background and basics, see e.g.