The derivative of 3x is 3. Whenever a mathematical function is defined by a constant multiplied by x, the derivative of the function equals the constant. Thus, the derivative of 2x is 2 and the derivative of 4x is 4.
The derivative of a function refers to the amount that the output of the function changes when the input changes by a tiny amount. When the function is graphed as a straight line, as is the case with f(x) = 3x, the derivative of the function is simply the slope of this line.
The field of mathematics that deals with derivatives is called differential calculus.