I looked at it this way:
Some number (let's call it y) is added to each term to get the next, right? So, let's take the first term and write and equation that states this:
2x - 3 + y = 5x + 1
Now, use inverse operations to solve for y. If you add 3 to both sides, and then subtract 2x from both sides, you're left with
y = 3x + 4
Now, to get the third term, we need to add y again, right? So, let's do some fancy substitutions.
2nd term + y = 3rd term:
5x + 1 + y = 4x - 7
Substitute the equivalent value of the 2nd term (from the first equation) and of y that we found:
5x + 1 + y = 4x - 7
2x - 3 + 3x + 4 + 3x + 4 = 4x - 7
Combine like terms on the left to get:
8x + 5 = 4x - 7
Use inverse operations to solve for x (on both sides: subtract 4x, subtract 5, and then divide by 4) and you get x = -3. Substitute this into each of the three original terms and check to see that you have an arithmetic progression (adding -5, which would be our y).
That was a more algebraic approach. I didn't do this as a first instict, but you can easily solve it with systems:
2x - 3 + y = 5x + 1
5x + 1 + y = 4x - 7
Subtract the second from the first and you eliminate the y. Your resulting equation is
-3x - 4 = x + 8
Inverse operations (on both sides: add 4, subtract x, and divide by -4) will lead to x = -3.
Always, substitute and check to ensure the accuracy of your answers!
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