Answer
By using the exterior angle property
The measure of exterior angle is equal to the sum of two opposite interior angles.
In ∠QSR is exterior angle.
∠SPQ and ∠SQP are the opposite interior angle of the exterior angle ∠QSR .
Thus
∠QSR = ∠SPQ + ∠SQP
As given in the diagram
∠QSR = 2x , ∠SQP = x
Put in the above
2x = x + ∠SQP
∠SQP = 2x - x
∠SQP = x
Thus
∠SPQ = ∠SQP = x
By using the property
if two angles are equal in a triangle than their opposite sides are also equal .
Thus
PS = SQ
Two sides of triangle are equal .
therefore Δ PQS is a isosceles triangle .