A linear function is an algebraic equation in which each term is either a constant or the product of a constant and (the first power of) a single variable.
The general equation for a linear equation is given by
y = mx + c
x, y, and c, are to the first power
The equations which don't satisfy the above definition show's that jimmy's conclusion is incorrect.
checking all the options,
Option A
y = 20 - 4x is linear
Option B
y= x²- 1 is not linear because x is to the power of 2, hence it is incorrect
Option C
y=5x³+9 is not linear because x is to the power of 3, hence it is incorrect
Option D
y= 2x(x-3) is not linear because x is to the power of 2, hence it is incorrect
Option E
y=15 is linear