Parallel lines

Since,

y : z = 3 : 7

So,

y = 3k and z = 7k (for some constant k)

Notice,

y + z = $180^\circ$

$\Rightarrow 3k + 7k = 180^\circ$

$\Rightarrow k = 18^\circ$

Therefore,

$z = 7k = 126^\circ= x$