There are 3 straight lines, with a total sum of 540° (180° x 3 )

They are made up of a,b,c,d,51°,38 and the 3 angles of the middle triangle. Recall the sum of angles in a triangle is 180°.

So a + b + c + d
= 540° - 180° - 51° - 38°
= 271°

Thank you..

Welcome.

Another approach to this question :

a + b + c + d
= Sum of angles of the 3 outer triangles
- 51° - 38° - the 3 unlabelled outer angles


Sum of angles of 3 outer triangles
= 180° x 3
= 540°

Notice the 3 unlabelled outer angles are actually angles of the big triangle. They add up to 180°

So a + b + c + d
= 540° - 51° - 38° - 180°
= 271°

Thanks for explaining.. much appreciated.:)