Triangle abc is an obtuse triangle with the obtuse angle at vertex b. angle a must be

less than 90°.
greater than 90°.
congruent to angle b.
congruent to angle c.

When you add up the measures of all three angles in a triangle,
they always add up to 180 degrees.  It doesn't matter how large
or small the triangle is, or how skinny or fat it is. It's a triangle thing.
The angles always add up to 180 degrees.

An 'obtuse' angle is an angle that measures more than 90 degrees
but less than 180 degrees, so it's possible to have 1 obtuse angle
in a triangle.  But if you have one, then that leaves less than 90 degrees
for the other two angles put together. 

So both of the other ones must be less than 90 degrees each.