中三數 coordinate~section formula

The coordinaates of M and N are (-10,-7) and (5,8) respectively. P is a point on MN extended such that MN:MP=5:8. Find the coordinates of P.

點搵MN:NP 個ratio係幾多...?
ratio可唔可以好似+ -數咁- -?
e.g NP=MP-MN
=8=5
=3
THX