Determine which of the following equations represents a linear function.
f(x)=x2+3f(x) = x^2 + 3f(x)=x2+3
f(x)=2xf(x) = \frac{2}{x}f(x)=x2
f(x)=5x−4f(x) = 5x - 4f(x)=5x−4
f(x)=2xf(x) = 2^xf(x)=2x