Step 1: Recall the standard form of a linear equation in two variables

A standard form of a linear equation in two variables is in the form ax+by = c

Step 1: Assume one piece of cable to be x and the other piece of cable to be y

Step 2: Write the first linear equation as x+ y = 84

Step 3: Assume x to be 18 metres longer than y

Step 4: Write the second linear equation as x = 18 +y

Step 5: Use the substitution (easy) method, write both the equations in x = form or y = form

x = 84 -y

x = 18 +y

Step 6: Write them across each other

84 - y = 18 +y

Step 7: Solve for y

84 - y = 18 + y

2y = 102

y = 51

Step 7: Substitute the value of y in any one of the equations to find the value of x

x = 84 - 51 = 33

So the length of one piece of cable is 33 m and the length of the other piece of cable is 51m