Find the length

$\large \displaystyle \text{Let }a \text{ be the width of the rectangle}\\ \large \displaystyle \therefore \text{Length is given by } 5a + 7\\ \large \displaystyle \implies \text{Perimeter of rectangle } = 2 \times (\text{length} + \text{width})\\ \large \displaystyle \implies 2 \times \left( a + 5a + 7 \right) = 86\\ \large \displaystyle \implies 6a + 7 = 43\\ \large \displaystyle \implies 6a = 36 \implies a = 6\\ \large \displaystyle \therefore \text{The length of the rectangle } = (5 \times 6) + 7 = 30 + 7 = \color{#3D99F6}{\boxed{37}}.$

Let the width be x and the length 5x+7.
Since the perimeter is 86 2x+2(5x+7)=86 2x+10x+14=86, 12x=86-14, x=72÷12, x=6. Since we know the length is 7 more than 5 times the width and we know the width is 6 we can now say, L=5(6)+7 or 37 units.

$\large \color{#3D99F6} \text{Just solve this equation where x is the width:} \\ \large \color{#20A900} 2(7 + 5x) + 2x = 86 \\ \large \color{#D61F06} x = 6 \\ \large \color{#3D99F6} \text{Using the fact that length is equal to '5 times the width and add 7' we can deduce that the length is equal to:} \\ \large \color{#69047E} \boxed{37}$