Answer:
Θ ≈ 103°
Explanation:
using the Cosine rule in Δ ABC
cosB =

where a is the side opposite ∠ A , b the side opposite ∠ B and c the side opposite ∠ C
here a = 42 , b = 78 and c = 57 , then
cos B =
=

=

=
, then
B = Θ =
( -
) ≈ 103° ( to the nearest degree )