An image that is 6 inches wide on a transparency is 18 inches wide on a screen. What is the scale factor?

It's larger on the screen so the factor is greater than 1.

Divide the size on the screen by the size on the transparency:

18 / 6 = 3

The scale factor is 3.

because linear equations are a lot simpler than quadratics equations, and linear equations formula is a lot easier to memorize. that's why i think they're harder.