In this question, you are given the NaOH volume but asked for concentration.
Don't forget that for every 1 mol of NaOH there will be 1 mol OH- ion, but for every 1 mol of H2SO4 there will be 2 mol of H- ion.
To neutralize you need the same amount of OH- and H+, so the equation should be:
OH-= H+
35.50cm3 * x*1= 25cm3* 0.2mol/dm3 *2
x= 10/35.5 mol/dm3= 0.2816/dm3