To Differentiate A linear Function from a non-linear one we have Several rules.
1: check the Variables exponent degree of the function if the exponent is zero or 1 of the function that's the function is linear.
2: if it's not 0 or 1 and Greater than 1 than it's non linear function also if the exponent is of any negative value like -1 or -2 than it's also a non linear function. The Given function is a non linear function because the 9xy value has two variables multiplied if we add the exponent values together which are 1+1 = 2 hence proved it's a non linear function. One More Thing a graph of non linear function will be always other than that of a straight line.
