Find the unknown angle in the following figure​

Ans is 18

we know that sum of all angles of triangle =180

so , 6x+3x+x=180

10x = 180

x = 180 ÷10

x = 18

now a = 6 × 18 = 108

b=3×18= 54

c= 108+54=162

180-162=18

hope it will help