In summary: F=mg (mass being 70kg+70kg+10kg) 150kg/(divied by) 9.8(g)=1470N(Newtons)For c, it's best to keep track of units, being lazy and dropping units mid-calculation can make it hard to figure out units later; it can also cost you some points depending on your teacher. 150kg was multiplied by 9.8 N Kg^-1 to convert 150kg to Newtons