You haven't told us whether 9 and 12 are the two legs of the triangle, or whether the 12yd is the longest side.
If the 9 and the 12 are the two legs, then:
C² = A² + B²
C² = 81 + 144
C² = 225
C = 15 yd
If the 12 is the longest side, then:
(12)² = (9)² + B²
B² = (12)² - (9)² = 144 - 81 = 63
B = √63 = 7.937yd (rounded)
