C, since both triangles are similar （angle a is shared, angle P and angle C are congruent, and Angle Q and Angle C are congruent from PQ and BC are parallel.
Then we can derive the equation for solving the similar polygons using ratios：
[tex] \frac{6}{x} = \frac{18}{x + 26} [/tex]
by using cross multiplication we can make the equation like this:
[tex]6x + 156 = 12x[/tex]
and we only have to solve for x which is:
[tex]156 = 12x[/tex]
[tex]x = 13[/tex]
