Ok, so the area of a rectangle is a=lw. So, you want to first replace everything with its value. It tells you that w=l-3 and that a=28. Everything is now with just one variable. You have 1 equation and you only have 1 variable, so you can solve (n variables = n equations you need to solve). Now your equation will be 28=l(l-3). Distribute the l, to make 28=l^2 - 3l. Move everything over and you get a simple quadratic, l^2 -3l -28. Use the AC method to find that 0=(l-7)(l+4). So l gets 2 values, 7 and -4. But a length cannot be negative, so your answer is 7. Hope that helped and hopefully I didn’t do anything wrong (the steps are definitely right, double check the numbers) :)