b$)$ Check whether the given schedules are conflict serializable or not

$6$

i$),S1$:$R_1(x),R_2(x),R_1(Y),R_2(Y),R_3(Y),W_1(x),W_2(Y)$

ii$),S2$:$R_1(x),R_2(x),R_2(Y),W_2(Y),R_1(Y),W_1(x)$

$20$ a$)$ What is two phase locking protocol? How does it guarantee serializability?

$8$