
In a absolute value graph the value of y is the same in -x and x.
Then if the coordinates (x,y) one point is (1, 8), the graph have another point in (- 1, 8).
To find the value of a you use one of the points (x,y) in the general form of absolute value:
( 1 , 8) x=1 f(x)= y=8

As:

You get:

Then:

The equation for the graph with points (1 , 8) and ( -1, 8) is:

a= 8