If A1 has the week number and year as a 3 or 4 digit integer in the format wwYY then the formula would be:
=INT(A1/100)*7+DATE(MOD([A1,100),1,1)-WEEKDAY(DATE(MOD(A1,100),1,1))-5
the subtraction of the weekday ensures you return a consistent start day of the week. Use the final subtraction to adjust the start day.