This is probably a really easy one to solve so hoping some kind soul can help me out!

I have a simple two variable data set which has a polyominal relationship and from which I've plotted out a curve based on the X,Y variables I've been provided.
It's very simple and here is a sample of that data:

X Axis Y Axis
9.67 8.6%
26.83 21.1%
43.64 28.7%
60.20 34.2%
74.62 38.1%
89.97 41.3%
107.30 44.5%
124.15 47.2%

I then have another sample set of X axis points which increases in increments of 5 as per below

Sample X axis
5
10
15
20
25
30
35
40

What I need to do is establish the relationship from my actual (real) data and apply it to the sample X axis data so that it projects a well estimated Y axis forecast. The chart trend line I've created is a 6 order polyominal and I've had no success copying that out and using it. I've therefore tried to use LINEST as I understand this is a much better way to go but have also come unstuck (also the fact that I'm using a mac seems to make array function harder too!)

I'm sure this is very easy to resolve and hoping someone kind can help me out