You did not post an equation. An equation must include an equals sign. Did you mean to post 7x + 9y = 28?
I am going to write an answer assuming you meant 7x + 9y = 28.
Without a second independent equation that uses one or both of these variables, you cannot solve for a single set of values. But you can solve for all values of x with respect to y or all values of y with respect to x.
7x + 9y = 28 (subtract 9y from both sides of the equation)
7x = -9y + 28 (divide both sides of the equation by 7)
x = -(9/7)y + 4
You now have a solution set for all values of x with respect to y
(x, -(9/7)y + 4)
You can select values for y and solve for x to have a definite solution set, but there is an infinite number of solution sets. Example:
Let y = 7/9
x = -(9/7)(7/9) + 4 = -1 + 4 = 3
So (3, 7/9) is a solution set. Try these two numbers in the original equation.
7(3) + 9(7/9) = 28
21 + 7 = 28
This equation is true, so (3, 7/9) is a solution set.
You can illustrate all solution sets with a linear graph by solving for y with respect to x.
7x + 9y = 28 (subtract 7x from both sides of the equation then divide both sides by 9)
y = (-7/9)x + 28/9
This is the y-intercept form of a linear equation. Its slope is -7/9 and its y-intercept is 28/9. A graph of the line illustrates all solutions for the given equation.