Answer:
77 N
Step-by-step explanation:
1st charge ( q1 ) = -80µC placed on x-axis at x = 0
second charge( q2) +50µC placed at x = 0.5 m 
3rd charge(q3) 4.0µC placed at x = 0.3 m
Determine the magnitude of the electrostatic force on the third charge 
calculate distance btw : 1st charge and 3rd charge = 0.3 m ( r1 )
distance btw : 2nd charge and 3rd charge = 0.5 - 0.3 = 0.2 m ( r2 ) 
hence the magnitude of electrostatic force on third charge 
F = [ Kq1q3 / ( r1 )^2 ] + [ Kq2q3 / ( r2)^2 ] ------ ( 1 )
where K = 9 * 10^9 N.m^2/c^2 
Insert given values into equation 1 
∴ F = 77.0N