The reduction in the cost of materials used in the manufacture of athletic shoes leads to an increase in supply (shift to the right as shown). The increase in the price of sports sandals leads to an increase in demand for athletic shoes since the two are substitutes (shown as a shift to the right). The shift out in demand and the shift out in supply both work to increase quantity. These shifts work in opposite directions with respect to price, thus we can't tell which of the two will be the stronger effect or if, as shown, they roughly cancel each other out.