the formula to find the perimeter in a rectangle is 2(L+W)=perimeter they gave us the perimeter so we substitute is into the equation: 2(L+W)=18 substitute the W by 1/2L u will end up with 2(L+1/2L)=18 distribute the 2 and u will get: 2L+1L=18 which is 3L=18 and L=6 we know that the width is half the length so W=1/2(6) so W=3