Explanation:
this is similar to the complete approach to find the midpoint. just instead of 1/2 we are looking for 3/10 of the distance.
the general approach is
A + (B - A)×factor
factor can be 1/2 or 3/10 or 7/10 or 24/29 or ...
so, we have
(-3, -6) + ((9 - -3)×3/10, (5 - -6)×3/10) =
= (-3, -6) + (12×3/10, 11×3/10) =
= (-3, -6) + (3.6, 3.3) = (0.6, -2.7)
the point 3/10 of the way from A to B is
(0.6, -2.7)