Use the compound interest formula
A = P (1 + r/n)^(nt).
Here A = unknown; B = initial amount = $300;
r = rate = 0.0218; n = 2 (2 compounding periods per year); and t = 1/2 (year).
Then A = $300 (1+0.0218/2)^(2*[1/2])
A = $300 (1.0218)^1 or A = $300(1.0218) = $306.54