Parallel lines mean equal angles, which means similar triangles. We can set up a ratio y like this. \(\frac {12}{5} = \frac {y}{3}\) using ratio of sides. Cross multiply, and you get 5y = 36. So y = 36/5.
Do the same for x. \(\frac {17}{x} = \frac {36/5 + 3}{6}\) . I don't like fractions over fractions, so imma make it a decimal to make it easier. \(\frac {17}{x} = \frac {10.2}{6}\) . Cross multiply and you get 10.2x = 102. Solving for x gets us x = 10