Two point charges are located on the y-axis as follow: charge q1==3.80x10^-9 C at y=0.600m and change q2= -2.50 x 10^-9 C at the origin (y=0). What is the total force exerted by these two charges on a third charge q3 = 6.00x10^-9 located at y= 0.400m.

Find the force that each of the charges have on the 3rd charge.

Force of 1st charge onto 3rd charge = \frac{Qq}{4\pi \epsilon_o r^2} or \frac{kQq}{r^2}

The 1st charge is negative (if I guess right. Because you put = instead of - maybe?) and the 3rd charge is positive. Hence the force is towards the right.

Force of 2nd charge onto 3rd charge = \frac{Qq}{4\pi \epsilon_o r^2} or \frac{kQq}{r^2}

The 2nd charge is negative and the 3rd charge is positive. Hence the force is towards the left.

Do your vector addition of the two forces to get the net force the 3rd charge experiences :)