It is given that
⇒∠SRT≅∠STR
m∠SRT=20°
So,⇒ m∠SRT=m∠STR=20°------{Using (1)]
In ΔSRT
⇒∠SRT+∠RTS+∠TSR=180°-----Angle Sum Property of Triangle
⇒20°+20°+∠TSR=180°
⇒∠TSR=180°-40°
⇒∠TSR=140°
⇒∠STU=∠TRS+∠RST-----[Exterior Angle of triangle is equal to sum of it's two interior opposite Angle]
⇒4x=20°+140°
⇒4x=160°
Dividing both side by 4, we get
x=40°
m∠STU=4×40°=160°