an example of a first-order model with three predictor variables is
y ^2 =β0+ β1x1 + β2x2 + β3x3 + ε
y = β0 + β1x1 + β2x2 + β3x3 + ε
y = β0 + β1x^2 + β3x3 + ε
y = β0 + β1x1 + β2x2 + ε