1.1 1-2x3, 2.1.2 sqrt(16+23), 2.1.3 [-12/-3 x (-2)2] + cbrt(11+16)

2.1

Step 1: Given expression
$21 - 1 \times 3$

Step 2: Perform multiplication first
$1 \times 3 = 3$
Substitute:
$21 - 3$

Step 3: Perform subtraction
$21 - 3 = 18$

18

2.12

Step 1: Given expression
$\sqrt{4} + 2^{2}$

Step 2: Evaluate square root
$\sqrt{4} = 2$
Evaluate exponent
$2^{2} = 4$

Step 3: Add the results
$2 + 4 = 6$

6

2.13

Step 1: Given expression
$\sqrt[3]{-8} + \sqrt[3]{18}$

Step 2: Evaluate first cube root
$\sqrt[3]{-8} = -2$
(since $(-2)^3 = -8$)

Step 3: Second cube root remains
$\sqrt[3]{18}$

Step 4: Add the results
$-2 + \sqrt[3]{18}$

-2 + \sqrt[3]{18}