The point (-1, 8) is on a line that has a slope of -3. is the point (4, -7) on the same line?

First you can use the formula and find your line so the formula is : y = mx + b and m is the slope and b is the y-intercept so now you should find b : 
8 = -3 (-1) +b ⇒ 8 = 3 + b ⇒ 5 =b  so now you have : y = -3x + 5 and if you want to know that the (4 , -7) is in this line or not you can say : 
-7 = -3(4) + 5 ⇒ -7 = -12 + 5 ⇒-7 = -7 so the point is on the same line :)))
i hope this is helpful 
have a nice day