Perform the following calculations: a) -81 + 0.79, b) 31 + (-16). Simplify: 2.1) 34, 2.2) 1.23, 2.3) 2.31.75. Calculate: 3.1) (62.4)1.3 - 173/85 + fourth_root(898)

$$-81 + 0.79 = -80.21$$

The result is:

$$\boxed{-80.21}$$

Adding a negative number is equivalent to subtracting.

$$31 + (-16) = 31 - 16$$

$$31 - 16 = 15$$

The result is:

$$\boxed{15}$$

2. Simplify

$$3 \times 3 = 9$$ $$9 \times 3 = 27$$ $$27 \times 3 = 81$$

The simplified value is:

$$\boxed{81}$$

$$1.2 \times 1.2 = 1.44$$ $$1.44 \times 1.2 = 1.728$$

The simplified value is:

$$\boxed{1.728}$$

$$2.3^{1.75}$$

Using a calculator:

$$2.3^{1.75} \approx 4.4170$$

The simplified value (rounded to four decimal places) is:

$$\boxed{4.4170}$$

3. Use the calculator to calculate

First term: $(62.4)^{1.3}$

$$(62.4)^{1.3} \approx 251.3852$$

Second term: $\frac{173}{85}$

$$\frac{173}{85} \approx 2.0353$$

Third term: $\sqrt[4]{898}$

$$\sqrt[4]{898} = 898^{1/4} = 898^{0.25} \approx 5.4886$$

$$251.3852 + 2.0353 = 253.4205$$ $$253.4205 - 5.4886 = 247.9319$$

The final calculated value (rounded to four decimal places) is:

$$\boxed{247.9319}$$