To express $9^{2x}$ in terms of base 3, let's start by rewriting 9 as a power of 3:

$9 = 3^2$

So, $9^{2x}$ can be rewritten as:

$9^{2x} = (3^2)^{2x}$

Using the power of a power rule, $(a^m)^n = a^{mn}$, we get:

$(3^2)^{2x} = 3^{2 \cdot 2x} = 3^{4x}$

Thus, $9^{2x}$ in terms of base 3 is:

$\boxed{3^{4x}}$

Would you like further clarification on this, or do you have any other questions? Here are some related questions that you might find interesting:

1. How can you convert other expressions with powers of 9 into base 3?
2. What are the rules for converting expressions from one base to another?
3. How do you simplify expressions involving exponents with different bases?
4. What is the general formula for converting $a^{bx}$ into a different base?
5. How do exponent rules apply to more complex algebraic expressions?

Tip: When converting expressions into different bases, always look for ways to express the original number as a power of the new base.