The slope was correctly found to be -4. There are several ways to find the y-intercept. One would be to use the information available to find the point-slope form of the equation for these values and then convert that to the slope-intercept form.
Using (25, 200) as our point and m = -4 as our slope, the point-slope equation would be y - 200 = -4(x - 25), which simplifies to y - 200 = -4x + 100. Converting to slope-intercept form, we get y = -4x + 300. So, our y-intercept would be 300 (that is, y = 300 when x = 0), as you also correctly obtained.
