First, let's simplify the expression (5a³)²:

(5a³)² = 5² (a³)²
= 25 a^(3*2)
= 25a⁶

Now, let's multiply b² by 25a⁶:

b² * 25a⁶ = 25b²a⁶

So the final answer is 25b²a⁶.