Atriangle has an area of 9^2−9−10. the base of the triangle is 3−5. what is the height of the triangle?

(a) What is the area of the triangle determined by the lines 
y =−1/2x + 9, y =10x, and the y-axis?
Thus we already know that 
Area of triangle= 1/2*b*h
Base=9

Now to find the height, we need the coordinates of the intersection so:

−1/2x+9=10x

x=6/7 <=height
y=60/7

So using the formula

Area=1/2*b*h= 1/2*9*6/7
=3.8571428

The answer that i got was 8/3