Calculate the %(m/m) concentration of 6.00 M H2SO4. The density of the 6.00 M H2SO4 solution is 1.34g/ml.

6.00 M means 6.00 mols H2SO4/L solution.

1 mol H2SO4 = 98 g; 6 mols approx 600 g but you need to do it more accurately.

That's 600 g/L. Use density to convert to ?g/?g solution, then convert that to g solute/100 g solution.