Two force problems

For your first question.

The net force will be zero if there is no acceleration! Newtons first law of motion.

same would apply to part b if it moves at constant velocity towards left.

For second question.

we know the mass and we know the force it can apply.
[tex]F=ma[/tex]

[tex]\frac{F}{m}=a[/tex]

[tex]\frac{15105}{5.5*10^7}=a[/tex]

[tex]a=2.75*10^-^4[/tex]

[tex]v=u+at[/tex]

velocity is 85KM/h which is 23.61 m/s

[tex]23.61=0+2.75*10^-^4t[/tex]

[tex]t=\frac{23.61}{2.75*10^-^4}[/tex]

[tex]t=85972 s[/tex]

23 hours 52 mins 52 seconds!

if the force applied by engine is 15105N.

if it is [tex]15*10^5[/tex] then the answer is 865.7 seconds