Y = mx + b
slope(m) = 7
(1,8)...x = 1 and y = 8
now we sub into the formula and find b, the y int
8 = 7(1) + b
8 = 7 + b
8 - 7 = b
1 = b
so ur equation is : y = 7x + 1 <===
Using the equation of the line
y -yo = m (x-xo)
We substitute xo,yo and the slope
y -5 = -8(x-1) .....> y = -8x +8 +5 = -8x +13
y = -8x +13
Sorry you dont have a picture so i can not help
He would have drank 22.4 litres of water in 14 days
