algebra
posted by chica .
By the end of your 5th French lesson, you have learned 20 vocabulary words. After 10 lessons you know 40 vocabulary words. Write an equation that gives the number of vocabulary words you know, y, in the terms of the number of lessons you had, x.

Assuming that the learning of vocabulary words increases in a linear pattern, (it does not), you have two ordered pairs.
(5,20) and (10,40)
Just like you would in grade 9 geometry, find the slope, then use y = mx + b to find the equation. 
Hello,
Did you try to answer yourself ?
x = number of lessons
y = number of vocabulary words
I help you : 5x = 20y ( 5th French lesson, you have learned 20 vocabulary words )
It's math
Good luck :) 
slope = (4020)/(105) = 4
so y = mx + b becomes
y = 4x + b
sub in (5,20)
20 = 4(5) + b
b = 0
so y = 4x is the equation.