Owing as an algebraic expression using x as your unknown variable
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If we have one zero of +5i, we also have to have its conjugate which is -5i. Therefore, the factors that we will FOIL to get that polynomial are (x+5i)(x-5i). That distribution results in 
. I'm sure by now you know that i squared is equal to -1, so -25(-1) = 25. So our polynomial is 