The intercept of the function is the value of y when x=0. In this particular case, when x=0, y=0, then the intercept is y=0.
The slope of the function must tell us what is the change in y when x changes. When x changes in one unit (from x=0 to x=1), y takes the value of y=-30, y=1x(-30). When x takes the value of x=2, y takes the value of y=-60, then y=2x(-30), and when x=3, y=3x(-30)=-90.
This means that y always takes the value of -30x. Then the slope is -30, which is the rate of change of y as x changes.
