what are complex numbers and why are they used?

what are complex numbers and why are they used?

part a- A complex number is a number that can be expressed in the form a + bi, where a and b are real numbers and i is the imaginary unit part b- The fundamental theorem of algebra states that every non-constant single-variable polynomial with complex coefficients has at least one complex root. part c- https://www.desmos.com/calculator/tdtyu3xsos