For b)
We will use the formula of Newton's second law

where F is the force, m is the mass and a is the acceleration. In this case we have
F=85N
m=75 kg
a=?
We will isolate the a

we substitute the values

c)
We will use the same formula that we use previously but in this case, we have the next data
F=-85N
m=55 kg

ANSWER
b) 1.13 m/s^2
c)-1.55 m/s^2