(a) Each triangle is formed by choosing 2 of those lines and the line x = 1. So if n < 4, n is one of 0,1,2,3 so there are 4C2 = 6 triangles
(b) Same as (a) except n can be one of 10 things. So 10C2 = 45 triangles can be formed.
(c)(i) We choose 2 of those triangles above, so 45C2 = 990.
(ii)...