OK, courtesy of some code I wrote some time ago, this task is not too difficult. However, do bear in mind that the user interface is not fool-proof. So, even coming from a technologically-challenged company you will have to do some work. Though, this is a non-programmatic solution so you don't have to write VBA code. Does that help?
Note that the x-values for both data sets have to be in ascending order.
Name (Insert | Name > Define..) the x-values of your first data set x1Vals. Name the y-values y1Vals. Similarly, name the x- and y-values of your 2nd data set x2Vals and y2Vals, respectively.
In the Visual Basic Editor...oh, no! Wait, we are using a non-programmatic solution. Scratch that.
Plot the two series so that you have an approximate idea of the x-values where the two intersect.
In some cell, enter that approx. value. Name that cell CommonXVal.
{Instead of typing the formulas below you may be better off with copy-paste steps}
In two adjacent cells, say, J8 and K8, enter the formulas
<pre>=MATCH(commonXVal,x1vals,1)</pre>
and
<pre>=MATCH(commonXVal,x2vals,1)</pre>
In the two cells just below these (i.e., J9 and K9), enter the formulas
<pre>=(INDEX(y1vals,J8+1)-INDEX(y1vals,J8))/(INDEX(x1vals,J8+1)-INDEX(x1vals,J8))*(commonXVal-INDEX(x1vals,J8))+INDEX(y1vals,J8)</pre>
and
<pre>=(INDEX(y2vals,K8+1)-INDEX(y2vals,K8))/(INDEX(x2vals,K8+1)-INDEX(x2vals,K8))*(commonXVal-INDEX(x2vals,K8))+INDEX(y2vals,K8)</pre>
If you didn't use J8 and K8 as in the example above, you will have to adjust the J8 and K8 references in the big messy formulas to refer to the cells you did use.
In K10, enter the formula <pre>=J9-K9</pre>Again, if you didn't use J9 and K9 for the formulas as in the example above, you will have to adjust the formula accordingly.
Now, run Goal Seek (though, personally, I prefer the more sophisticated Solver) by selecting Tools | Goal Seek... Set the 'Set Cell' to K10, the 'To Value' to 0 (that's zero), and the 'By changing cell' to CommonXVal.
Click OK and you should have the exact result.
