I have $57 and I make $10 a week. How much will I have after 3 weeks?

57 + 10(3) = $87

How much will I have after x weeks?

57 + 10x

Since x is an unknown number, I can't calculate any further. In math, though, this looks like...

y = 10x + 57

y = mx + b, where m = the slope and b = the y-intercept.

You could graph your situation. At 0 weeks, he has $57, thus a point at (0,57). The next point will be up 10 and over 1 to (1,67) because after 1 week I'll have $67.