Sam solved the following equation for $x$. Find the line that has a mistake or state that there is no mistake:

Equation: $8{ x }^{ 2 }(3x+7)+2x=24{ x }^{ 3 }+50{ x }^{ 2 }$

Step 1: $24{ x }^{ 3 }+56{ x }^{ 2 }+2x=24{ x }^{ 3 }+50{ x }^{ 2 }$

Step 2: $56{ x }^{ 2 }+2x=50{ x }^{ 2 }$

Step 3: $6{ x }^{ 2 }+2x=0$

Step 4: $2x(3x+1)=0$

Step 5: $3x+1=0$

Step 6: $3x=-1$

Step 7: $x=-\cfrac { 1 }{ 3 }$