At (x, y, z) = (2, 9, 6) the function is approximately
f(x, y, z) = f(2, 9, 6) + 2·2·(x-2) + 2·9·(y-9) + 2·6·(z-6)
f(x, y, z) = 121 + 4(x-2) +18(y-9) +12(z-6)
For your given numbers, the value of f(x, y, z) is approximately
f(2.03, 8.98, 5.99) ≈ 121 + 4(.03) +18(-.02) +12(-.01)))
= 121 +0.12 -0.36 -0.12 = 120.64