Is 7,849 a reasonable answer for 49*49? Why or why not

No, for a couple of reasons.
1. The result will be less than 50*50 = 2500
2. The result will end in 1, because 9*9 = 81.
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49*49 = 2401

No, because the right answer is 2,401. If you still think that is a wrong answer it really depends on the context of the answer, for example some classes will have you use the 3 significant figures (first 3 non-zero numbers) others have a very complicated way of doing significant figures. Also, some might want you to put it to the power of 10 for example in the answer from before you could write it down as 2.401x 10^3.