indifference curve slope from utility function in the economics book that I'm reading right now it is written that this utility function:
$$u(x_1,x_2) = 2x_1 + x_2$$
yields indifference curves with a slope of $−2$.
Could someone please explain me how they found the $-2$? 

Initially, I was thinking that they derivated the utility function in respect of $x_1$ and $x_2$ but this would give $2$ instead of $-2$. Thank you very much for your help
Hint: First find the total derivative of $u(x_1,x_2)$, set it to zero as utility does not change along an indifference curve, then solve for $dx_2/dx_1$