A2.0 milliliter sample of naoh solution is exactly
neutralized by 4.0 milliliters of a 3.0m hci solution.
what is the concentration of the naoh solution?

V ( NaOH) = 2.0 mL / 1000 => 0.002 L 
M( NaOH) = ?

V ( HCl) = 4.0 mL / 1000 => 0.004 L
M(HCl) = 3.0 M

number of moles HCl:

n = M x V

n = 3.0 x 0.004

n = 0.012 moles of HCl

Mole ratio:

HCl + NaOH = NaCl + H2O

1 mole HCl 1 mole NaOH
0.012 moles HCl ?

moles NaOH = 0.012 x 1 / 1 => 0.012 moles

M = n / V

M = 0.012 / 0.002

= 6.0 M

hope this helps!

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.