What is angle y and why ? ? when b is 90

You have to sit calmly and quietly and look over this one for a few minutes,
until you see what's going on.

When the top triangle got flopped over to make the bottom one,
the 12-cm side on top made another 12-cm side on the bottom.

Also, the short 6-cm piece made another 6-cm piece hooked onto it,
and when those are put together, they make 12-cm all together.

So now, look at the whole outside triangle that you have, after the two
inside ones are put together.  The top slanty side is 12-cm, the bottom
slanty side is 12-cm, and the straight up-and-down side is also 12-cm.
The new big triangle is an equilateral triangle !

What do you know about an equilateral triangle ?

-- All three sides are the same length.
-- All three angles are the same size.

If all three angles are the same size, how big is each one ?

Hint:  The three angles of every triangle add up to 180 degrees.

So angle-y must be 1/3 of 180 = 60 degrees.