Answer: 180g
Step-by-step explanation:
No of moles= volume ×molarity/1000
Volume is given as 750ml
Molarity is given as 6M
Substitute to get the mole
No of moles = 750×6/1000
No of moles= 4.5moles
Remember that 
No of moles also = mass given/molar mass 
Molar mass of NaOH
Na= 23
O=16
H=1
Molar mass = na +o+h
Molar mass =23+16+1
Molar mass=40g/mol
Substitute 
No of moles = mass given/molar mass 
Mass = 4.5×40
Mass = 180g
The mass of NaOH needed is 180g