Using the given data, calculate the rate constant of this reaction.

According to the attached picture of the complete question that you have missed:
when Rate = K [A]^x * [B]^z
So to get the rate we have to get X & Z
first, we choose two value of [A] which [A] changed and [B] still constant
that appears in trial 1 & 3 SO, we will use Rate 1 & rate 3 and [A1] &[A3] and we can neglect [B] as it is constant:
Rate3/Rate1 = ([A3]/[A1])^x
0.0852/0.0213 = (0.62/0.31)^x
4 = 2^x ∴ X = 2 
- second, we have to choose two value of [B] which [B] changed and [A] still constant and that appears in trail 1&2, we will use the rate 1 &2 and [B1] & [ B2] and we can neglect [A] as it is constant:
Rate 2 / Rate 1 = ([B2]/[B1])^z
0.0213/0.0213 = (0.68/0.34)^z
1 = 2^z ∴ Z= zero 
SO Rate1 = K [A]^2[B]^0
   0.0213 = K * (0.31)^2 *1
∴ K = 0.222 m^-1.S^-1

They are all decomposition reactions.