We are given the equation x plus 9 equals 21. Our goal is to find the value of x that makes this equation true. In this equation, x is an unknown value we need to determine, while 9 and 21 are known numbers.
Think of an equation like a balanced scale. Both sides must remain equal. Whatever operation we perform on one side of the equation, we must also perform on the other side to maintain balance. This is the fundamental principle we'll use to solve our equation.
To solve for x, we need to isolate it on one side of the equation. Since we have x plus 9, we need to eliminate the plus 9. We do this by subtracting 9 from both sides of the equation. This gives us x plus 9 minus 9 equals 21 minus 9.
Now we have simplified our equation to x equals 21 minus 9. We just need to perform this subtraction to find the value of x. 21 minus 9 equals 12. Therefore, x equals 12. This is our solution.
It's always good practice to verify our solution. We do this by substituting our answer back into the original equation. We replace x with 12 in the equation x plus 9 equals 21. This gives us 12 plus 9 equals 21. Calculating the left side, 12 plus 9 equals 21. Since both sides equal 21, our solution is correct.
There are alternative ways to think about this problem. First, we can ask ourselves what number plus 9 equals 21. Counting up from 9 to 21, we find it's 12. Second, we can directly think of this as 21 minus 9, which equals 12. All these approaches lead us to the same answer, confirming our solution.
Let's practice with two more examples. First, x plus 5 equals 18. To solve, we subtract 5 from both sides: x equals 18 minus 5, which is 13. Checking: 13 plus 5 equals 18. Second, x plus 7 equals 25. Subtracting 7 from both sides: x equals 25 minus 7, which is 18. Checking: 18 plus 7 equals 25. Both solutions are correct.