how much concentrated 18 m sulfurid acid is needed to prepare 250 mL of a 6.0M solution?

You can do it as one equation, but I will break it down into parts so you can better see what is going on.

First, find out how many moles of H2SO4 are in 250 ml of 6M acid:

0.250 L x 6mole/L = 1.5 moles

Now, find out what volume of 18M sulfuric acid contains this quantity of acid:

1.5 moles/18 moles/L = 0.0833L

So, take 83.3 ml of the 18M acid and dillute with water to 250 ml