It is given in the question that:

AB || CD and CD || EF

∠y : ∠z = 3 : 7

Now,

∠x + ∠y = 180^o (Angles on the same side of transversal) ---------- (i)

And,

∠O + ∠z = 180° (Linear pair) -----------------------(ii)

And,

∠y = ∠O (Corresponding angles) -------------------(iii)

From (ii) and (iii)

∠y + ∠z = 180^o

Now, according to question,

∠y = 3w and ∠z = 7w

3w + 7w = 180^o

10w = 180^o

w = 18^o

Hence,

∠y = 3 × 18^o = 54^o

And,

∠z = 7 × 18^o = 126^o

Now, from (i)

∠x + ∠y = 180^o

⇒ ∠x + 54^o = 180^o

⇒ ∠x = 126^o