Explanation:
the circumference of a whole circle is
2×pi×r
with r being the radius. which is in our case a.
so,
2×pi×a
the part arc of just a sector of the circle is just the angle/360 part (as the whole circle is 360°).
we therefore know
27×pi = 2×pi×a × 60/360 = 2×pi×a × 1/6 = pi×a × 1/3
3×27×pi = pi×a
81×pi = pi×a
a = 81