Answer: 4(x^2 + 4)
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Step-by-step explanation:
The terms 4x^2 and 16 have the factor 4 in common. We can pull factor out using the distributive property
4x^2 + 16 = 4*x^2 + 4*4 = 4(x^2 + 4)
So that's how 4x^2+16 turns into 4(x^2+4)
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further info:
If you want to factor over the complex numbers, then x^2+4 would factor into (x+2i)(x-2i)
though it's not clear if your teacher wants you to do this or not