What is the equivalent expression to 7x-8y+9x

That expression can be simplified by combining like terms.
Like terms are terms that have the same variable(s) and the same exponent(s).
The like terms are 7x and 9x.

You can combine like terms by adding the coefficients (the number in front of the exponent) together and then tacking on the variable at  the end.
7x + 9x = 16x

Add 16x back to the term that can't be combined (-8y), and the simplified expression is 16x - 8y