Oh that’s what you need help with. That’s easy. All you need to do is add 20+74=94, then you need to subtract 180 ( which is all of the sides lengths added together) and 94 which equals 86. So the missing angle is 86.The formula/expression is
A+B+C=180 degrees. You have 3 acute angles because 20,74 and 86 are less than 90 and 90 is half of 180. Any triangle that uses a 90 degree angle is called a right triangle. Since all of your angles are acute you are an acute triangle.
Thank you so much now I see how to find angles. I could not have done it without you.