The sum of the interior angles of a triangle is 180 degrees.
1.) Translating the problem into equation:
7:11:12 = 180 degrees
7x+11x+12x = 180 degrees; Where x = missing number
30x = 180
30x/30 = 180/30 divide both sides by 30
x = 6
2.) Substitute the values:
7x+11x+12x = 180
7(6)+11(6)+12(6) = 180
42+66+72 = 180
180 = 180
3.) Therefore, the angles should be 42 degrees, 66 degrees and 72 degrees.
4.) See attached drawing of triangle