Determine the x-coordinate of the vertext of thee parabola by making use of the fact that at the vertex the slope of the tangent is zero

After reading 80% of her e-mails in her inbox, danette still has m unread e-mails. which of the following expressions could represent the number of e-mails danette had in her inbox before she started reading? choose 2 answers: (choice a) 5m (choice b) 1.8m (choice c) 80m (choice d) m/0.8 (choice e) m/1−0.8