hi

Firstly , resolve the forces into their vertical and horizontal components respectively . Also , taking upwards and right as positive .

Vertical : 8 sin Θ -4

horizontal : 8 cos Θ -2

Since the resultant force (F') is horizontal , the vertical component would be 0 , 8 sin Θ -4 =0 , Θ = 30 degree .

F'=8 cos 30 -2 =4.93 N

F=ma , 4.93=4a

a=1.233 m/s^2