3.2.14 Lines On a Plane

Condition: in the rectangular system of coordinates on the plane of the side \ (a b \) and \ (b c \) the triangle \ (a b c \) lie on straight \ (x-2 y+18 = 0 \) and \ (x-y-1 = 0 \), respectively. The heights of the triangle intersect at the point \ (h (1; 2) \). Find the coordinates of the vertex \ (a \).