b)
\begin{align} ΣF_y=0 =&\cssId{Step1}{F_{Ay} + F_{By} + F_{Cy}=0}\\[3px] &\cssId{Step2}{0 + F_B * sin(α) + F_{Cy}=0}\\[3px] &\cssId{Step3}{F_{Cy}= -F_B * sin(α)}\\[3px] \end{align}
In the first step we make the summation of forces in the y-axis
In the second step we express F
By
as F
B
multiplying the sinus of the angle (doing trigonometry)
In the third step we isolate F
Cy
to one side of the equation and everything else to the other side so that you can solve the equation