Answer :

Alright! Let's solve the given problem step-by-step.

### Part 1: Solve for [tex]\( x^2 - 81 = 0 \)[/tex]

1. The equation given is:
[tex]\[ x^2 - 81 = 0 \][/tex]

2. To solve for [tex]\( x \)[/tex], we can add 81 to both sides:
[tex]\[ x^2 = 81 \][/tex]

3. Next, take the square root of both sides:
[tex]\[ x = \pm \sqrt{81} \][/tex]

4. Since [tex]\( \sqrt{81} = 9 \)[/tex], we get:
[tex]\[ x = \pm 9 \][/tex]

So, the solutions for [tex]\( x \)[/tex] are [tex]\( x = 9 \)[/tex] and [tex]\( x = -9 \)[/tex].

### Part 2: Calculate [tex]\( a = \sqrt[3]{x^2} \)[/tex]

1. We need to find [tex]\( \sqrt[3]{x^2} \)[/tex]. We already know that [tex]\( x \)[/tex] can be [tex]\( 9 \)[/tex] or [tex]\( -9 \)[/tex].

2. Calculate for [tex]\( x = 9 \)[/tex]:
[tex]\[ \sqrt[3]{9^2} = \sqrt[3]{81} \][/tex]

3. Calculate for [tex]\( x = -9 \)[/tex]:
[tex]\[ \sqrt[3]{(-9)^2} = \sqrt[3]{81} \][/tex]

4. Therefore, in both cases, we get:
[tex]\[ a = \sqrt[3]{81} \][/tex]

### Part 3: Calculate [tex]\( b = \sqrt[3]{81} \)[/tex]

1. The value for [tex]\( \sqrt[3]{81} \)[/tex] is a cube root of 81.

2. Calculate the cube root of 81:
- Let's express 81 as [tex]\( 3^4 \)[/tex].
- The cube root of [tex]\( 3^4 \)[/tex] is [tex]\( 3^{4/3} \)[/tex].

3. Approximating, we get:
- [tex]\( 3^{4/3} \approx 4.326 \)[/tex] (approximately).

Hence, we find:
[tex]\[ b = \sqrt[3]{81} \approx 4.326 \][/tex]

### Summary

1. The solutions for [tex]\( x \)[/tex] are:
[tex]\[ x = 9 \text{ and } x = -9 \][/tex]

2. The value of [tex]\( a \)[/tex]:
[tex]\[ a = \sqrt[3]{81} \approx 4.326 \][/tex]

3. The value of [tex]\( b \)[/tex]:
[tex]\[ b = \sqrt[3]{81} \approx 4.326 \][/tex]

So, the final results are:
[tex]\[ a = \sqrt[3]{81} \approx 4.326 \][/tex]
[tex]\[ b = \sqrt[3]{81} \approx 4.326 \][/tex]