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Hi, I'm Mike G and I'll be assisting you. You'll need a 2 wire voltage tester to identify the hot wires. I suspect that there are more than one baseboard heaters, that's why you have that many wires. Let me know if you have a teeter.
Could be the reds are the hots. Best to be sure and test the wires.
You'll test each pair of wires for 240V. The connect the hot set to the line terminations and both of the others to the load terminations. The T-stat will control both heaters.