The lines intersect at point L, so all angles have a vertex (middle letter) L so there is no angle TRL
Straight angle is a line with dot-vertex, so the straight angles are ∠RLS and ∠TLU.
∠TLS is less than 90° then it is acute angle (∠SLT is the same angle). ∠ULR is vertex angle to ∠TLS, so it's also acute angle.
Two angles adding to straight angle mean that they are both right angles or one is acute and the second is obtuse. ∠TLS is acute so ∠RLT is obtuse (they adding to ∠RLS) and ∠SLU is obtuse (they adding to ∠TLU). ∠ULS is the same angle as ∠SLU.
