We are tasked with determining the expression at the top of the pyramid. Each brick's expression is formed by adding the two expressions in the bricks below it.

Step 1: Adding the bottom row expressions to get the middle row

The bottom row has the expressions:
$5x^2 + 2$, $4x$, and $2x^2 + 5x$.

Middle-left brick:

Add the first two expressions:
$(5x^2 + 2) + 4x = 5x^2 + 4x + 2$

Middle-right brick:

Add the second and third expressions:
$4x + (2x^2 + 5x) = 2x^2 + 9x$

So the middle row consists of:
$5x^2 + 4x + 2 \quad \text{and} \quad 2x^2 + 9x$

Step 2: Adding the middle row to get the top brick

Now, add the two expressions from the middle row:
$(5x^2 + 4x + 2) + (2x^2 + 9x)$

Combine like terms:
$5x^2 + 2x^2 + 4x + 9x + 2 = 7x^2 + 13x + 2$

Final Answer:

The expression replacing the question mark is:
$\boxed{7x^2 + 13x + 2}$

Would you like me to break any step down further or clarify anything?

Related Questions:

1. How can we generalize this method for any pyramid of expressions?
2. What happens if one of the terms in the base row is a constant instead of an expression?
3. How would the solution change if subtraction were used instead of addition between the bricks?
4. How can we check if the resulting expression at the top is factorizable?
5. What are practical applications of such pyramids in algebra?

Tip:

When simplifying algebraic expressions, always group like terms systematically to avoid errors.