18:12 is NOT equivalent to 3:4.
For two ratios to be equivalent, the smaller ratio needs to equal the larger one when both the numbers are multiplied by a certain value.
Since we know that 3*6 = 18, let's multiply 4 by 6 to see if it's equivalent.
4*6 = 24. Because 4*6 does not equal 12, we know that the ratios are not equivalent.