We see that point A and C have the same y vo-ordinate i.e 4. Let us take AC as the base. NOw the height is given by the distance between the base and side B. The y co-ordinate of side B is -6. Thus the height will be (4+6)= 10 units. now area of the triangle is 1/2*base*height=35. i.e 1/2*base*10=35 i.e base=7. which means the distance between  points A and C must be 7. x coordinate of c is 5. thus the x coordinate of A is -2. as the distance between -2 and 5 is 7 units!