Answer:
(4a - 9)(4a + 9)
Explanation:
Every value in the equation has a square root
The square root of 16 is 4
The square root of a^2 is a
The square root of 81 is 9.
To factor you split the equation by the square roots. For 81, since the original value is negative, one of the roots must be negative so when you multiply it back out to the original, it stays the same.
You can check your answer this way too.
(4a -9)(4a + 9)
= 16a^2 + 36a - 36a - 81
= 16a^2 - 81
I hope this helps!