internally, u need to make one of the numbers in the ratio negative(usually the smaller number)
i.e.
internally
(-1,3) (5,8)
A : B
= 3 : -2
working out:
x= ((-1 x -2) + (5 x 3))/(3+ -2) = 17/1 =17
y= ((3 x -2) + (8 x 3))/(3+-2)= 18/1 = 18
therefore point= (17,18)...