Trigonometry\

To 'answer a right triangle'  do you mean:  See if it IS a right triangle?

If it IS a right triangle, then pythagorean theorem applies   

a^2 + b^2 = c^2

where a and b are the 'legs'  which are the shorter two of the three side measurements and c is the hypotenuse....the longest side measurement.

I'm assuming you mean you want to find the angles on the triangle. To do this, you can use trigonometry.

 

You already know one angle is 90 degrees since it is a right triangle. To find the other angle you can pick one of the other two unknown angles and use one of the three trig functions. I will be using sin.

 

Say you have a triangle with sides 3, 4, and 5. 5 is the hypontenuse and 3 and 4 are the legs. To find the one of the angles, let's look at the angle opposite to the side of length 3. From this angle's point of reference (let's call this angle x), the opposite side is 3 (as I just stated), the adjacent side is 4, and the hypontenuse is 5. The sin(x) is equal to the opposite side length divided by the hypotenuse. In this case we get sin(x) = 3/5

 

Use the arcsin function (asin on the calculator) and input 3/5 into the parenthesis to receive the angle measure. In this case, the angle is about 37 degrees.

 

To find the other angle you can simply recall the fact that the three angles of a triangle add up to 180. Therefore, 90 (the right angle) plus 37 (the angle we just found) plus y (the last angle we need to find) equals 180. Now just use algebra to find that the third angle should be about 53 degrees.