1) I'm sorry, but I can't understand your post. Please reply using single-line formatting. If this is one equation and it contains fractions, please use the "slash" and grouping symbols to make your meaning clear. For instance, "1/3x" might mean:
. . . . .1/(3x)
...or:
. . . . .(1/3)x
When you reply, please include the instructions. Thank you.
2) Solve P = s1 + s2 + s3 for s3.
If the numbers are coefficients, so you actually mean "1s + 2s + 3s", then, yes, they can be combined, being "like" terms, to get "6s". Then solve for "s" (not "3s") by dividing through by "6".
If the numbers are exponents, so you actually mean "s<sup>1</sup> + s<sup>2</sup> + s<sup>3</sup>", then they cannot be combined, but then the "solving" part of the instructions doesn't make a whole lost of sense.
If the numbers are subscripts, so you actually mean "s<sub>1</sub> + s<sub>2</sub> + s<sub>3</sub>", then I have no idea on what basis these three different variables might be "combined" into a fourth variable, "s<sub>6</sub>", unless there is a whole lot more to this exercise that wasn't included.
Please reply with clarification. Thank you.
Eliz.