Force is variable so average force = (Final force+initial force)/2=(600+400)/2=500 N
resistance ia constt so net force acting=500-90=410 N
work done =F.x=410.15=6150J = Change in KE = KEf-KEi
KEi=0(given, as object at rest initially)
so (1/2)(mass)(v^2)=6150
mass=3075
hence
v^2=6150*2/3075=4
so v=2