Use the substitution property of equations to solve the following equation for $x$ if $y=3 x$. \[ 4 x+3 y=39 \]

Step 1 ：We are given that \(y = 3x\). We can substitute this into the equation \(4x + 3y = 39\) to solve for \(x\).

Step 2 ：Substituting \(y = 3x\) into the equation gives us \(4x + 3(3x) = 39\).

Step 3 ：Simplifying the equation gives us \(13x = 39\).

Step 4 ：Dividing both sides of the equation by 13 gives us \(x = 3\).

Step 5 ：Final Answer: The solution to the equation is \(x = \boxed{3}\).