Pankaj gets ice cream in a cone that is 6 inches tall and has a radius of 2.5 inches. Ice cream completely fills the cone and forms a
hemisphere on top of the cone. What is the volume of the ice cream to
the nearest cubic inch

You can use the C. HL Theorem to prove the triangles congruent.

To use the HL THeorem to prove the triangles congruent, you have to check for three conditions. 
1) The triangles are both right triangles.
2) Their hypotenuses are congruent
3) They have one pair of congruent legs.

The squares at the angle of the triangle indicate the triangles are right triangles. Since the triangles in the picture share one side, the hypotenuse, the hypotenuses of the triangles are congruent. The triangles also have one pair of congruent legs. Since all three conditions of the HL Theorem are met, the triangles are congruent.