The exponential and the logarithm are inverses of each other. A rule I was taught in school was "B.A.N" or B^A = N, where B is the base of the log, A is the part after the equals (the "answer") and N is the number you are applying the log to.

Thus, B=7 , A=2x , N=25.
so the log 25 = 2x (not sure how to type this so i put the 7 on the next line)
7
Type that into your calculator and halve the answer and you get x.
Hope this helps.

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