Solution: Conservation of Momentum

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This is a very easy problem, you should've solved this instantly in your head. Because it is an elastic collision, the relative velocity of approach is equal to the relative velocity of recession. Which means that the total magnitude of velocity is conserved in the system and because all balls have equal masses, the third ball, blue one will have the difference of velocities of other two balls.
v_blue = 10m/s - 2m/s = 8m/s

Answer: 8