Question 754886
It cannot be factored with real numbers.  However if you've
studied imaginary numbers and your teacher accepts such 
factoring, which is permissible in more advanced algebra,
then do it this way: 
`
16x²+81 = 16x²-(-81) = 16x²-(81)(-1) = 16x²-81(i²) = 16x²-81i² = (4x-9i)(4x+9i)
`
Edwin