f'(x) = lim(Δx->0)(f(x + Δx) - f(x))/Δx
f(x) = 3 - 7x
f'(x) = lim(Δx->0)(3 - 7(x + Δx) - (3 - 7x))/Δx = lim(Δx->0)(3 - 7x - 7Δx - 3 + 7x))/Δx = lim(Δx->0)(- 7Δx))/Δx = -7
