If w(x)=3x+7 , then w(cow)=3cows+7 , and w(mouse)=3mice+7 and w(4)=3fours +7.
This last one is the one you have to work with.
w(4) = 3 fours + 7
w(4) = 3(4) + 7
w(4) = (12) + 7
w(4) = 19
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