Problem given
We know that AC=45
AB=BC= 4x-9
And we also know that A,B and C are collinear
Solution
From the info given we can set up the following equation:
AC= AB+BC
Replacing we got:
45= 4x-9 +(4x-9)
If we simplify the terms in the right we got:
45= 8x -18
And if we solve for x we have:
45+18= 8x
63= 8x
And solving for x we got:
x= 63/8