I wonder if you mean right triangle?
There are quite a few possibilities if all you know is one angle is 25 degrees and another is less than 90 degrees.
"Also, how would I figure out if it is possible to have a triangle with sides 5, 7, and 12"
Cut three paper strips of length 5, 7, and 12 inches.
See if you can make a triangle of it.
Again, I wonder if you mean a right angle?