Open the brackets: x+15x-100=10x-40, then gather the x's together on the left and the constants on the right: 6x=60, so x=10. When gathering terms together from each side of the equals sign, remember to change the sign from + to - and - to + when moving across the equal sign. So 10x on the right becomes -10x on the left when collecting the x terms on the left, and -100 becomes 100 when collecting the constants on the right: x(1+15-10)=100-40.