Thank you for showing your work. I think you have failed to take into account the added water from the steam AND that the final T is NOT 100 C.
So I think you have three items at play here. Steam condenses at 100 to liquid water (which adds to the mass of the water) and that heat is Hvap x mass steam. Mass steam is the unknown here.
Then you have the mass of the water from the steam x sp.h. x delta T (which is Tfinal-Tinitial = 76-100). Finally you have the original 250 g H2O being heated up from 21 to 76 (or Tf-Ti = +55). The + signifies that heat is being added. The steam condenses to give off heat so that portion of the equation is negative and the 100 water then cools to 76 (Tf-Ti=76-100=-24) so it is negative also. I hope this helps.