The sum of all the angles made at a point on a line where n lines meet, then the sum of (n+1) angles is 180°, as here, 2 lines meet at the center of the semicircle and so, 3 angles are here.So, by using this property we will be having the equation ;
[tex]{:\implies \quad \sf x^{\circ}+7x^{\circ}+x^{\circ}=180^{\circ}}[/tex]
[tex]{:\implies \quad \sf 9x^{\circ}=180^{\circ}}[/tex]
[tex]{:\implies \quad \sf x=\dfrac{180^{\circ}}{9}=\bf \boxed{20^{\circ}}}[/tex]
This is the required answer
