The input for the equation y = 2x + 3 can be any real number, as x is the independent variable in the equation. If you want to find specific values of y for given inputs of x, you can substitute those values into the equation. For example, if x = 2, the input is 2, and you can find the corresponding output, y, by substituting x = 2 into the equation: y = 2(2) + 3, which equals 7. Therefore, when the input is 2, the output is 7.