What is the x-intercept of the line with this equation 1/3x+y=−15 ? enter your answer in the box. (, 0)

-45 

X intercept is just like the Y intercept except you're looking for the intersection of the line where Y = 0, instead of X=0.  So take the expression, substitute the value 0 for Y, and then solve for X. So
1/3x + y = -15
1/3x + 0 = -15
x = -45

So the X intercept is -45.