This kind of problem is actually quite simple. I will give you anothe problem like this and solve it for you so that you can see what is going on here.
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Let's take the problem \(\displaystyle 3y+6y=81\).
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First, it is important that you undertsand that x is just a variable. That means that ANY number could be put in it's place. But, since this equation has an = sign in it, there is a specific value for y (which is the same as using x). So, you're problem is trying to solve for the variable.
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Since we have 3y and 6y on one side with a plus between them, we know to add the two together. The way we do this is just by adding the 3 to the 6 and placing that in front of the y. It will look like this.
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\(\displaystyle 3y+6y=9y\)
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Now, we have to set that 9y equal to 81.
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\(\displaystyle 9y=81\)
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So, now we are one step away from having our final answer.
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9y is the same thing as saying 9 times y. So, to get rid of y, you have to divide it. That makes the left side just y by itself. Does that mean that y=81? No. Whatever you do on one side of a = sign, you ALWAYS do on the other. So, we divide 81 by 9.
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\(\displaystyle y=\frac{81}{9}\)
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This should be easy. We know that 81 divided by 9 is 9. So, we now have our answer.
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\(\displaystyle y=9\)
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It's that simple. So, now that you know how to do it, try your original problem on your own and tell us whatcha get!
Good luck!
