In order to determine the coordinates of A, use the following formula for the area of a triangle:
A = b·h/2
where b is the base of the triangle, and it is equal to the length of the segment CB = 8 units. h is the height of the triangle and it is equal to the length of segment AC.
Due to you know what is the area of the triangle (A = 28 units²), solve the formula for h, and replace the rest of the parameters, as follow:
h = 2A/b
h = 2(28 units²)/(8 units)
h = 7 units
Then, segment AC = 7 units. Due to the point A is on the y-axis, it is necessary that the coordinates of A are A(0,7)