The mass of the truck remains constant during this acceleration (that's a sensible assumption). So we can use the equation for Newton's Second Law with a constant mass:
#F = ma#
#F# is the resultant force. The question does not specify any resistive forces acting on the truck, only the motive force (of 4,200 N). So in this case we will ignore the resistive forces and the resultant force is equal to the motive force.
Rearrange the equation for #a# and substitute the values into the equation to solve:
#a = F/m = 4200/2000 = 2.1 m .s⁻²#