You know the line goes through (0, 2) and (4, 0). That means it’s got a slope of –1/2 and a y-intercept of 2. So start by putting it into slope-intercept form.

y = mx + b
y = (–1/2)x + 2

Now all you need to do is manipulate the equation into the form the question asks for.

y + (1/2)x = 2
(1/2)x + y = 2

There, that lines up with Ax + By = C. But the question tells you that A = 6! What do we need to multiply the equation we found by to get 6 for the coefficient of x? We need to multiply it by 12.

12((1/2)x + y) = 12(2)
6x + 12y = 24

So there you have it: B = 12.