# two bloock are arranged

Treat the two blocks as a single system. The two external forces in the direction of motion is gravity on the 1.42kg block and the friction on the 3.09kg block. So the net force and acceleration are related as $F_{net}=1.42g-f=(1.42+3.09)a$.
Use $v_f^2-v_i^2=2a\Delta x$ to find $a$. Then you can find the frictional force with the previous equation.