Thank you for the opportunity to help you with your question!
When you are swimming (or rowing or flying or whatever), against / with a current, your total speed is caused by two things : the rate you are propelling yourself (what you could do in still water) and the rate the current is moving. When you are swimming with the current, the current increases your speed. When you are swimming against the current, the current reduces your speed.
So, lets call the rate that you can swim in the water S and the rate the current moves C.
Against the current: S - C = 1.5 m/s
With the current: S + C = 3.5 m/s
And now we can solve those two equations like any other system of equations. Start by adding the two equations together.
2S = 5 m / s
Solve for S
S = 5 / 2 = 2.5 m/s
You could then also solve for C if you needed to.
2.5 - C = 1.5
C = 1 m/s
So we have determined that the swimmer would be able to swim at 2.5 m / s in still water.
Please let me know if you need any clarification. I'm always happy to answer your questions.