Question 44114
All the angled inside a triangle add up to 180 degrees.
Let the angle we dont know in triangle (a) be called "A"
48+69+A=180
A=180-69-48=63 degrees
Now list all the anlges in triangle (a): 48, 63 and 69 degrees

Now do the same for triangle (b):
63+48+B=180
B=69 degrees
Now list all the anlges in triangle (b): 48, 63 and 69 degrees

Therefore triangles (a) and (b) are similar.

The unknown angle in triangle (c) is:
C=180-63-78=39 degrees
There are no 39 degree angles in triangles (a) and (b), so (c) is not similar to the other triangles.

I hope this helps 
P.S. I am trying to start up my own homework help website. I would be extremely grateful if you would e-mail me some feedback on the help you received to adam.chapman@student.manchester.ac.uk