Answer
At $y=0$:
$F_y = 0$
At $y=1~m$:
$F_y = -12~N$
At $y=2~m$:
$F_y = -48~N$
Work Step by Step
$U = 4y^3~J$
We can find an expression for $F_y$:
$F_y = -\frac{dU}{dy}$
$F_y = -12y^2~N$
We can find $F_y$ at $y = 0$:
$F_y = -12y^2~N$
$F_y = -(12)(0)^2~N$
$F_y = 0$
We can find $F_y$ at $y = 1$:
$F_y = -12y^2~N$
$F_y = -(12)(1)^2~N$
$F_y = -12~N$
We can find $F_y$ at $y = 2$:
$F_y = -12y^2~N$
$F_y = -(12)(2)^2~N$
$F_y = -48~N$