Q :How Can We Increase the Concentration of a 2 M Nitric Acid Solution to 6 M Nitric Acid Using 15 M Nitric Acid???
Ans : grin emoticon 
Step 1
You Must Know the Density of Liquid solution
You Must know the percentage purity of Liquid solution 
after that you can easily prepapere you liquid solution
Step 2
densiry of 15 M HNO3= 1.42 g/mL
Percentage purity = 68 %
Step 3 :
mass of HN03= 68 % x100=68g
Moles of HN03= Mass /mol.w8 =68/63.01=1.079 Mole
Mass of Water =mass of Solution-mass of HN03
= 100-68= 32 g
Remember the density of water is 1 g/mL thus 32 g of water equal to 32 mL so we can get the molarity of HCl solution
Molarity of HNO3= Mol/volume = 1.079mol/0.032L=1.074 M
You can use this formula for the dilution calculation ( this calculation is made if you want to prepare 6 M HCl 100 mL)
M1V1=M2V2
1.074XV1=6MX100
V1=558.6 mL
STEP 4:
if you have used the mL of HNO3 for 2M solution in 100 mL water .
then remaining mL can use i think you can easily understand