All angles in a triangle add up into 180. Therefore, if one of them is 90 (the right angle), and the other is 55, you can work out the last angle by doing 180 - 90 - 55 = a.
a = 35 degrees.
Angle b can be worked out by knowing the fact that a straight line has an angle of 180 degrees. We know that angle a is 35, so angle b will be 180 - a, so you do 180 - 35, which is 145 degrees.
b = 145 degrees.
Angle c is the same as angle a (they share a line), so it's 35 degrees too.