You are correct about point a), the mass of the remaining uranium-238 is indeed 1.26 kg.
Now for point b). The trick here is to know that you don't get 1 gram of lead-206 for every 1 gram of uranium-238 that decays, but rather 1 mole of lead-206 for every 1 mole of uranium-238 that decays.
Since you're left with 1.26 kg, the mass of uranium that dcayed is
Convert this value to grams
Now use uranium-238's molar mass to see how many moles decayed
This means of course that you've produced
This translates to a mass of
Rounded to three sig figs, the answer is