Help With Factoring

[StanTheMan]

I came across this problem on a math worksheet while I was getting rid of my useless school stuff from the previous year, and noticed this problem that I hadn't done. It ask me to factor (4x^2)+49 and I have no idea how to do it. I know the answer is (2x+7i)(2x-7i) because I looked it up on Wolframalpha, but I have no idea how to actually do the problem.