I am writing a program in Excel and I have a matrix chart or an array
(15x11). What I am looking to do is to find the y-value which corresponds to
a known x-value but the x-value is dependent upon a value entered by the user.

In other words, I have a chart with 15 sets of x-values and depending upon a
value which is entered by the user, the exact value determines which set of
x-values is used to find the corresponding y-value.