$-5x \times 8{x}^{2}-5x \times \left( -4x \right)-5x \times 7$
Calculate the product$-40{x}^{3}-5x \times \left( -4x \right)-5x \times 7$
Multiplying two negatives equals a positive: $\left( - \right) \times \left( - \right)=\left( + \right)$$-40{x}^{3}+5x \times 4x-5x \times 7$
Calculate the product$-40{x}^{3}+20{x}^{2}-5x \times 7$
Calculate the product$-40{x}^{3}+20{x}^{2}-35x$