(MC^2 + C√P)^2 wouldn't give you that result though, because you have to FOIL.
Instead you'd get M^(2) C^4 + 2MC^(3)√P + PC^2
And that's not even the correct formula. It's
E^2 = (mc^(2))^2 + (pc)^2
You can't just naively apply a square root unless one of the terms is vanishing (momentum for a stationary mass, giving E = mc^2, or rest mass for a massless particle, giving E = pc = hf).
The way to remember this is that it's equivalent to the Pythagorean theorem, A^2 + B^2 = C^(2).