This took me a few minutes to calculate - I don't blame you for not being able to calculate it. I'm in year 13 so after a year's worth of practising chemistry on an A-level standard, you will be able to spot key points.
1) 1 mole of bromine-79 ions weigh 79 grams. Convert the grams to kg and divide by Avogadro's constant (6.02*10^23).
2) Use the information about the same kinetic energy to solve the question. KE = 1/2 mv^2.
You have the mass of 1 bromine-79 ion, the TOF and the length of the tube. Using the TOF and the length of the tube, you can calculate speed (v=s/t) Substitute the values of v and s in to calculate its velocity, and substitute the velocity of it into KE = 1/2 mv^2, as well as the mass of 1 ion of bromine-79.
Now you use the fact that the EK's are the same, so the EK of bromine-81 is the same. You can do what you did in part 1 to calculate the mass (in kg again) of the bromine -81, and rearrange the EK to give you v. Afterwards you can plug the value of v back into the equation of (t=s/v) and you should have an answer of TOF.
If you have any questions, let me know! I'll be happy to help (or try to).