A triangle has two angles with measures 43 and 51.

What is the third angle in Euclidean geometry?

What is the third angle in Spherical geometry?

In Euclidean geometry, the angles of a triangle add to 180, which means the remaining angle has a measure of 86 degrees.

In spherical geometry, it's impossible to know the measure of the remaining angle precisely without knowing any more info about the triangle. The sum of the interior angles is between 180 and 540 degrees, so the missing angle is somewhere between 87 and 445 degrees.