C1 OCR solomon paper increasing functions

Hi can anyone help me with Question 3 on the OCR Solomon paper H

paper--https://a9497d7f220e174d38d014b2b1d86a68bedc42a8.googledrive.com/host/0B1ZiqBksUHNYNExoNEtSS1ViN2c/for-OCR/Solomon%20H%20QP%20-%20C1%20OCR.pdf

Mark scheme -- https://a9497d7f220e174d38d014b2b1d86a68bedc42a8.googledrive.com/host/0B1ZiqBksUHNYNExoNEtSS1ViN2c/for-OCR/Solomon%20H%20MS%20-%20C1%20OCR.pdf

I cant understand why dy/dx has been solved greater than or equal to 0 do you not show that a function is increasing my solving dy/dx> 0

It's a mistake. The function is stationary at $x=-3$ and $x=\frac{1}{3}$.

It's a mistake. The function is stationary at $x=-3$ and $x=\frac{1}{3}$.