We know that for this equation, 360 is going to be our total. If the skyway's main span is 30 meters shorter, we will be subtracting 30 from whatever our x value is, as shown here:
360 = x - 30
To solve for x, we must do the opposite of the operation we see. The opposite of subtracting 30 is adding 30. So we must add 30 to both sides of our equation.
360 + 30 = 390
So,
x = 390
This means that the length of the main span of the dames point bridge is 390 meters.
