Get right-hand side of equation

I am calling the function mnewton(0=expr, alpha, %pi/4) to get the root of a rather complex equation expr.

%(i1)   mnewton(0=expr, alpha, %pi/4)
%(o1)   [alpha=0.678193754078621]

I need to apply another function to this result (e.g. sin) and then want to plot it. Just linking the functions does not work:

%(i2)   sin(mnewton(0=expr, alpha, %pi/4)[1])
%(o2)   sin(alpha=0.678193754078621)

This is because the expression alpha=0.678193754078621 is not a number. How do I convert alpha=0.678193754078621 to just 0.678193754078621?

I can't just copy the numerical value and add it manually as I want to plot this and my expr will have a different root for each y.