** note above the value is expressed in DEGRES not angles, I apologize.
Now, to convert from Cartesian to Polar, consider the following.
for this example, I am assuming we have the value of X in cell B1 and the value of Y in cell B2
the formula for r is =SQRT(POWER(B1,2)+POWER(B2,2))
and the formula for tetha is =DEGREES(ATAN(B2/B1))
Note the function Power(A,b) is used to elevate A to the power of b, hence Power(B1,2) will yield "B1 squared".
Another note is that ATAN (arctangent function) returns a value in RADIANS, so we have to convert those to degrees using the function =DEGREES(r).
Hope this helps!!