3 times the difference of a and b is equal to 4 times c

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3 times the difference of a and b is equal to 4 times c

The difference of a and b:
a - b

3 times the difference of a and b:
3(a - b)

4 times c:
4c

The phrase is equal to means an equation. So we set 3(a - b) equal to 4c:
3(a - b) = 4c