To solve for m;
39 = 3m -12
39+12 = 3m -12 + 12 (add 12 to both sides)
51 = 3m
51 ÷ 3 = m
17 =m
m =17
Hope that helps, let me know if you don't understand something :D
Answer:
Step-by-step explanation:

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Ratio and proportion
height :legnth of shadow=56:49 (man's shadow)
we assume the ratio is the same for the tower thing (obelisk)
so
56:49=x:28
conver tto fraction
56/49=x/28
times both sides by 28
1568/49=x
32=x
answer is 32ft