Hello,melody and anon!
I think melody made a mistake.(9pi/12-10pi/12 is in inside of the parentheses)
y=18.5+6sin((9(pi/12)) - (10(pi/12)))
y=18.5+6sin(9pi/12-10pi/12)
y=18.5+6sin(-pi/12)
y=18.5-6sin(pi/12)
y=18.5-6sin(pi/4-pi/6)
$${\mathtt{y}} = {\mathtt{18.5}}{\mathtt{\,-\,}}{\mathtt{6}}{\mathtt{\,\times\,}}\left(\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{sin}}{\left({\frac{{\mathtt{\pi}}}{{\mathtt{4}}}}\right)}{\mathtt{\,\times\,}}\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{cos}}{\left({\frac{{\mathtt{\pi}}}{{\mathtt{6}}}}\right)}{\mathtt{\,-\,}}\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{cos}}{\left({\frac{{\mathtt{\pi}}}{{\mathtt{4}}}}\right)}{\mathtt{\,\times\,}}\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{sin}}{\left({\frac{{\mathtt{\pi}}}{{\mathtt{6}}}}\right)}\right)$$
$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{sin}}{\left({\frac{{\mathtt{\pi}}}{{\mathtt{4}}}}\right)} = {\frac{{\sqrt{{\mathtt{2}}}}}{{\mathtt{2}}}}$$ $$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{cos}}{\left({\frac{{\mathtt{\pi}}}{{\mathtt{6}}}}\right)} = {\frac{{\sqrt{{\mathtt{3}}}}}{{\mathtt{2}}}}$$ $$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{cos}}{\left({\frac{{\mathtt{\pi}}}{{\mathtt{4}}}}\right)} = {\frac{{\sqrt{{\mathtt{2}}}}}{{\mathtt{2}}}}$$ $$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{sin}}{\left({\frac{{\mathtt{\pi}}}{{\mathtt{6}}}}\right)} = {\frac{{\mathtt{1}}}{{\mathtt{2}}}}$$
$${\mathtt{y}} = {\mathtt{18.5}}{\mathtt{\,-\,}}{\mathtt{6}}{\mathtt{\,\times\,}}\left({\frac{{\sqrt{{\mathtt{6}}}}}{{\mathtt{4}}}}{\mathtt{\,-\,}}{\frac{{\sqrt{{\mathtt{2}}}}}{{\mathtt{4}}}}\right) = {\mathtt{18.5}}{\mathtt{\,-\,}}{\frac{{\mathtt{3}}{\mathtt{\,\times\,}}{\sqrt{{\mathtt{6}}}}}{{\mathtt{2}}}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{3}}{\mathtt{\,\times\,}}{\sqrt{{\mathtt{2}}}}}{{\mathtt{2}}}}$$
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