First, let's expand both terms:
(a^2 - 5)^2 = (a^2 - 5)(a^2 - 5)
= a^4 - 5a^2 - 5a^2 + 25
= a^4 - 10a^2 + 25

(2a + 3)^2 = (2a + 3)(2a + 3)
= 4a^2 + 6a + 6a + 9
= 4a^2 + 12a + 9

Now, let's substitute these back into the original equation:

(a^4 - 10a^2 + 25) - (4a^2 + 12a + 9) = 0

Simplify the equation:

a^4 - 10a^2 + 25 - 4a^2 - 12a - 9 = 0

Rearrange the terms:

a^4 - 14a^2 - 12a + 16 = 0

This is the simplified form of the given equation.