Subtract the first row from the second row, leaving other entries unaltered, so only the second row becomes [0 -11 0].

This doesn’t change the determinant since we may split the determinant after the alteration into determinants of two matrices, one of which is the original matrix, then the other one would have its first and second row differing only by a factor of -1 and hence its determinant is 0.

Use cofactor expansion on the second row, noting that 0 in the second row contributes nothing, we immediately get a=6sqrt(2) or -6sqrt(2)