Solve by substitution method 5x+y=18 and y=4x

Asap! i'm not good math and i'm lost.

Write an equation of the line that passes through 9,2 and is parallel to the line y=5/3x+9

Which function can be used to model the monthly profit for x trinkets produced? f(x) = –4(x – 50)(x – 250) f(x) = (x – 50)(x – 250) f(x) = 28(x + 50)(x + 250) f(x) = (x + 50)(x + 250)