So, step two, 1+2y? Equivalent to a coefficient, unchanged.

The derivative of the latter exponential term follows the basic function derivation rule and is a compound function.

(e^f(x))'=e^f(x)×f'(x)=e^(x？ +y？ )×(x？ +y？ )'=e^(x？ +y？ )×2x

That's how the coefficient of 2 comes from.