Answer
$17 s^{-1}$
Work Step by Step
Given, $E_{a}= 56.8 kJ/mol=56800 J/mol$
$A= 1.5\times10^{11}/s$
T= (25+273.15)K= 298.15 K
Recall that $k=Ae^{-\frac{E_{a}}{RT}}$
Then, $k= 1.5\times10^{11}s^{-1}\times e^{-\frac{56800J/mol}{8.314J mol^{-1}K^{-1}\times298.15K}}$
$=17 s^{-1}$