Solve using substitution. \( \begin{array}{l}x+y=3 \\ 8 x+9 y=20 \\ (\square, \square)\end{array} \)

To solve the system of equations using substitution, start with the first equation \(x + y = 3\). You can express \(y\) in terms of \(x\): \[ y = 3 - x. \] Next, substitute this expression for \(y\) into the second equation \(8x + 9y = 20\): \[ 8x + 9(3 - x) = 20. \] Now, distribute \(9\): \[ 8x + 27 - 9x = 20. \] Combine like terms: \[ -x + 27 = 20. \] Subtract \(27\) from both sides: \[ -x = -7. \] Multiply both sides by \(-1\): \[ x = 7. \] Now, substitute \(x = 7\) back into the expression for \(y\): \[ y = 3 - 7 = -4. \] Thus, the solution to the system of equations is \( (7, -4) \).