Question: Closest fit position of line

What value of 'a' gives the best fit for the line y=3x+a to pts[[3,4],[5,6],[21,16]] ?

I'm getting errors using LinearFit from the Statistics package but maybe I'm not using it right.

pts1 := Vector([3, 5, 21]);
pts2 := Vector([4, 6, 16]);

L := LinearFit(3*x+a, pts1, pts2, a);

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