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.

You wonder:

"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?

Bye.