α, β, γ are the lowercase Greek letters alpha, beta, and gamma in that order. They are the first three letters of the Greek alphabet.
The angle 50 and angle gamma at the top combine to form a 90 degree angle.
50 + γ = 90
γ= 90-50
γ = 40
The double tickmarks on the triangle on the right tells us that this triangle is isosceles, and therefore angle γ = β = 40, since the base angles are congruent. The base angles are opposite the congruent sides.
Lastly, we use the idea that the three inside angles of any triangle will add to 180 degrees.
α + β + γ = 180
α + 40 + 40 = 180
α + 80 = 180
α = 180 - 80
α = 100
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Summary
α = 100
β = 40
γ = 40