How to calculate mixing ingredients to a certain fat percentage?
As a matter of arithmetic, what you want is for the weighted average fat content to be 9%.  We can say
42X + 3Y = 9
where X and Y are the fractions of the whole that are from each of the two components.
X+Y=1
as the parts add up to the whole, so
42X+3(1-X)=9
leading to
39X=6
This means that a mixture of 2/13 42% fat plus 11/13 3% fat will make 9%.
It's important to be consistent.  If your fat percentage of both ingredients is by weight, then work in weight throughout, if by volume, work in volume.
To put this into practice, let's say both are by weight, and we want 100g in total.
2/13×100g = 15.4g
11/13×100 = 84.6g
You can't measure that accurately, so round to 15g of 42% fat and 85g of 3%, and you'll have 100g of 9%