the distributive property
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Using the distributive property which is a(b+c) is equal to ab + bc
In this problem 0 will be a, 3.7x will be b, and 4.21 will be c
So now we have 0(3.7x - 4.21) is equal to 0 times 3.7x minus 0 times 4.21
which equals 0 minus 0 which is 0
P.S if you meant 0(3.7 times -4.21) the answer would still be zero, however it wouldn't be the distributive property.