Explanation:
95°
Since, PQ || RS and QR is transversal.
\(\therefore\angle\text{PQR}=\angle\text{SRQ}\) [Alternate interior angles]
\(\Rightarrow\angle\text{SRQ}=85^\circ\)
Also, ST || QR and RS is transversal.
\(\therefore\angle\text{SRQ}=\angle\text{RST}\) [Alternate interior angles]
\(\Rightarrow\angle\text{RST}=85^\circ\)
Now, \(\angle\text{RST}+\text{a}=180^\circ\) [Liner pair]
\(\Rightarrow \text{a}= 180^\circ-\angle\text{RST}\)
\(\Rightarrow \text{a}=180^\circ-85^\circ\)
\(\Rightarrow \text{a}=95^\circ\) \([\because\angle\text{RST}=85^\circ]\)