Answer:
y = 7°
Step-by-step explanation:
By interior angle sum theorem of a triangle.
[tex]51 \degree + 87 \degree + 6y = 18 \degree \\ \\ \implies 138 \degree+ 6y = 18 0\degree \\ \\ \implies 6y = 180 \degree - 138 \degree \\ \implies 6y = 42 \degree \\ \\ \implies y = \frac{42 \degree}{6} \\ \\ \huge \red{\implies y =7 \degree} \\ \\ [/tex]
