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15=4h-21
We move all terms to the left:
15-(4h-21)=0
We get rid of parentheses
-4h+21+15=0
We add all the numbers together, and all the variables
-4h+36=0
We move all terms containing h to the left, all other terms to the right
-4h=-36
h=-36/-4
h=+9
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