Use the equation to find the slope: m=y2-y1/x2-x1 and plug in each of the y coordinates into the numerator and each of the x coordinates in the denominator
m=-2-3/4-8
-5/-4
m=5/4
Plug in the slope and one of the ordered pairs into y=mx+b. I’ll use (8,3)
3=5/4(8)+b
3=10+b
-7=b
Equation: y=5/4x-7
Remember that c is the initial height. Since we the rocket is in a 99-foot cliff, c=99. Also, we know that the velocity of the rocket is 122 ft/s; therefore v=122
Lets replace the values into the the vertical motion formula to get:

Notice that the rocket hits the ground at the bottom of the cliff, which means that the final height is 99-foot bellow its original position; therefore, our final height will be h=-99
Lets replace this into our equation to get:


Now we can apply the quadratic formula

where a=-16, b=122, and c=198


or


or


or

Since the time can't be negative, we can conclude that the rocket hits the ground after 9 seconds.
Sorry this is really sloppy but hope this helps!