An equation of a nonlinear function and provide two inputs to evaluate?

y=x2+3

x1=1

x2=2

y(x1) = 1*1+3 = 4

y(x2) = 2*2+3 = 7

x2/x1 = 2, While y2/y1 = 7/4 !=2, and thus the function is nonlinear.