You combine like terms so 5x + 3x= 8x and 2y + 4y = 6y so it is 8x + 6y
180 * 365 = 65700 litres/year
65700 / 1000 = 65.7 m^3/year
0.9122 * 65.7 = £59.932 for water used
28.20 + 59.93 = £88.13 for water meter annually
£88.13 < £107 so yes, he should have a water meter.
f(x) = 7x + 13
f(-3) = 7(-3) + 13
f(-3) = -21 + 13
f(-3) = -8