let

v = the boats speed in still water, v>0

c = 2 mph the speed of the current

d = 154 miles

downstream speed: v + c

t1 = the time traveling downstream

upstream speed: v - c

t2 = the time traveling upstream

the return trip upstream took 8 hours longe

t2 = t1 + 8

since speed = distance/time

v + c = d/t1

v + 2 = 154/t1

t1 = 154/(v + 2)

v - c = d/t2

v - 2 = 154/t2

t2 = 154/(v - 2)

lets plug t1 and t2 in t2 = t1 + 8

154/(v - 2) = 154/(v + 2) + 8

by solving the equations we find and consider only the positive roots since v>0

v = 9 mph

click here to see the step by step solution of the equation

Click to see all the steps

the speed of Marissa in still water is 9 mph.