Having problem looking up a function other than math.pow for c#

This is what I have come up with so far.
When I am compiling, an error is showing up in the return z; it says "cannot simplicity convert type 'double' to 'string'. I need a second pair of eyes because I can't see what I am doing wrong



using System;

class AI_Assignment_1

{
static string PowerProblem(double x, double y)
{
double z;
if (y == 2)
{
z = x * x;
}
else if (y == 3)
{
z = (x * x) * x;
}
else if (y == 4)
{
z = ((x * x) * x) * x;
}
else if (y == 5)
{
z = (((x * x) * x) * x) * x;
}
else if (y == 6)
{
z = ((((x * x) * x) * x) * x) * x;
}
else if (y == 7)
{
z = (((((x * x) * x) * x) * x) * x) * x;
}
else if (y == 8)
{
z = ((((((x * x) * x) * x) * x) * x) * x) * x;
}
else if (y == 9)
{
z = (((((((x * x) * x) * x) * x) * x) * x) * x) * x;
}

return z;
}
static void Main()
{
Console.Write("Enter Base Number: ");
double a = double.Parse(Console.ReadLine());
Console.Write("Enter Exponent Number: ");
double b = double.Parse(Console.ReadLine());
Console.WriteLine(PowerProblem(a, b));
}

}

yes, I understand your point. I know if i used math.pow it would easily solve it, but he doesn't want us too use it, hmmm. Give me a moment to rethink this and I will let you take a look at it.
I truely appreciate you taking the time to help me.

Juan

Thank you for the advice of starting with powers of two. I think that is the direction that i will start. Another person here has suggested that I tweak my search term to math.pow alternative for C# and I thing that solved the issue as to get a better search result.

I am sorry for coming across as a lazy person. I only started programing a couple of weeks ago and I lack the experience as to were I can find help through the internet. My professor wants us to find help through the internet, because in the real world we will eventually need to rely on our sources to be able to figure any troubleshooting that we may need fixing.

Thank you for taking time from your day to answer my question.

Sincerely

Juan