The arc length (s) is given in terms of the radius (r) and central angle (θ) by
s = r*θ . . . . . . . where θ is in radians
For your arc, the length is
s = (15 ft)*(π/4) ≈ 11.78 ft
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45° can be converted to radians by multiplying by π/180°.
45° * (π/180°) = π*(45/180) = π/4 . . . . radians