Problem 1
We're given that angle A = angle B. Also that angle B = angle C.
By the transitive property, we can say angle A = angle C.
So basically all three angles are equal to one another. Let's call that unknown angle x.
For any triangle, the three angles always add to 180, so,
x+x+x = 180
3x = 180
x = 180/3
x = 60
That proves angles A,B, and C are each 60 degrees. This triangle is considered equiangular since all angles are the same. Furthermore, the triangle is also considered equilateral meaning all sides are the same. The term equilateral is more widely used, so I'd go with that term if you could only pick one.
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Problem 2
As mentioned in problem 1, the three angles of a triangle add to 180
A+B+C = 180
A+B+90 = 180
A+B+90-90 = 180-90
A+B = 90
This shows A and B are complementary angles. Complementary angles by definition add to 90 degrees.
