"Work and struggle and never accept an evil that you can change." [André Gide]
If you cannot use macros you might be tempted to use the worksheet function TREND for interpolation. Of course you can't just feed all y-values, x-values (if there are more than 2) and the new x for the unknown y because TREND will just be piecewise accurate.
So you need to identify the surrounding x-values (or the first two resp. the last two in case of extrapolation) for the new x with their corresponding y-values first and then you feed them to TREND.
I don't like this approach and I won't recommend to use it but if you do want to, use it correctly: