Question 269618
Try this:
{{{A = L*W}}} Substitute A = 216, and L = W+6
{{{216 = (W+6)(W)}}}
{{{216 = W^2+6W}}} Rearrange to:
{{{W^2+6W-216 = 0}}}  Factor.
{{{(W+18)(W-12) = 0}}} so...
{{{W = -18}}} or {{{W = 12}}} Discard the negative solution.
{{{W =12}}} and...
{{{L = (W+6)}}}
{{{L = 18}}} so...
The length of the cutting board is 18 inches and the width is 12 inches.