You are given triangle DEF and it is a right triangle. You are also given line segment FE which is 12 and line segment DE which is equal to 5 You are asked to find DF. In order for you to solve this you have to know the pythagorean theorem for right for right angled triangles.
Let us say that line segment DE is at the base of the triangle and line segment EF is 12, you can find the length of line segment DF.
DF² = EF² + DE²
DF² = 12² + 5²
DF² = 169
DF = √169
DF =13