A suitable probability app or calculator can tell you
p(21.3<x<24.1) ≈ 2.14%
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The z-values associated with your limits are
... (x - μ)/σ = (21.3 - 15.7)/2.8 = 5.6/2.8 = 2
... (24.1 - 15.7)/2.8 = 8.4/2.8 = 3
You can look up the probabilities associated with these values in a table of the standard normal probability distribution, or using a suitable probability app or calculator.
... p(z < 2) ≈ 0.97725
... p(z < 3) ≈ 0.99865
... p(z < 3) - p(z < 2) = p(2 < z < 3) = 0.99865 - 0.97725 = 0.02140
