We start from the concentration of OH- given then multiply this to the desired volume of the solution to obtain the number of moles OH- needed.
0.100 mol OH- / L soln ( 0.250 L) = 0.025 mol OH-
We then use the relation of OH ions with the OH ions formed when Sr(OH)2 • 8 H2O is dissolved in water to calculate the number of moles of the compound needed. Finally we multiply the value with the molar mass given to obtain the number of grams needed.
0.025 mol OH- (1 mol Sr(OH)2 • 8 H2O / 2 mol OH-) (265.76 g / 1 mol) = 3.322 g Sr(OH)2 • 8 H2O needed