Line 1 = -4i + j +3k + s(i + 5j - 2k)

Line 2 = 3i - 7j + 9k + t(2i - 3j + k)

The lines are skew.

Question: Find the position vector of the point C on line 2 such that ∠ABC = 90°.

Annnd...i have no idea how...I've tried random things but nothing works.

The answer is -7i + 8j + 4k

Any help or prods in the right direction would be appreciated.

Thank-you very much!