The perimeter of a rectangle is the sum of all its sides. As a formula, this is
P = w + w + l + l = 2w + 2l = 2(w + l),
where P is perimeter, w is the width, and l is length.
Let's apply this formula to your question and substitute given information, then solve for l.
P = 2(w + l)
10x + 6 = 2(3x - 3 + l).
10x + 6 = 6x - 6 + 2l
4x +12 = 2l
l = 2x + 6