You find the x-intercept by replacing y with zero. 1/3x+0=-15. 1/3x=-15. Next, you have to divide each side by 1/3. In order to divide a fraction, you have to flip it , then multiply. That means you are now multiplying by 3. -15 times 3 is -45, which means x=-45. That means your answer is (-45,0)