What is the solution to the following 21+x=13

Which statement best explains whether the table represents a linear or nonlinear function? input (x) output (y) 2 5 4 10 6 15 8 20 it is a linear function because the input values are increasing. it is a nonlinear function because the output values are increasing. it is a linear function because there is a constant rate of change in both the input and output. it is a nonlinear function because there is a constant rate of change in both the input and output.