You know that the angles in a triangle always add up to 180 degrees, and you know angle a is 35 degrees and angle C is 90 degrees (as indicated by the little square). You must add 90 + 35 which would be 125 degrees,and then subtract 180-125. Basically, for this to be a triangle this last unknown angle must add up with the other two to give you 180, the only possible answer is 55; which would be C
