Question

find a polynomial of least degree(having real coefficients) with zeros: 5, -2, 2i

Answer 1

multiply this out:

(x-5) (x^2+4)

I suspect you copied the problem wrong, it should be zeroes at 5, -2i, 2i

Answer 2

Unless there is the implicit assumption that if 2i is a root, so is -2i, so the polynomial would be

(x-5)(x+2)(x^2+4) = x^4 - 3x^3 - 6x^2 - 12x - 40