Not sure what it is that you're trying to compute but it looks similar do a distance calculation.
a | \ c
c = √a² + b²
If that's the case you need to move the addition of b*b inside the parentheses of the square root call. I honsestly think you're using way too many parentheses here. You're probably confusing yourself.
// Is this what you want?
c = sqrt(a * a + b * b);
If you are going to do it this way you should use atan2(y,x) rather than atan(x): you will get an answer in the correct quadrant (otherwise you will be wrong 50% of the time) and you won't crash when a = 0.
Given what you are doing you would be better with complex numbers: see the advice given to you in your other post.
What you have written in your answer line is also patently wrong: that is not a polar form. You want something like, e.g., cout << "Multiplication: " << c << " exp( " << d << " i)" << endl;
or, if you must, cout << "Multiplication: (modulus, argument) = (" << c << ", " << d << " )" << endl;
For complex numbers in polar form leave the angle in radians - there is no need for the extra multiplying factor of 57.2958, which would be incorrect once it is inside the complex exp().