You cannot add two fractions whose denominators disagree — the pieces are different sizes, so the tops are not countable. The fix is always the same: find the smallest expression both denominators divide into, then rename each fraction so it sits on that expression. That smallest expression is the least common denominator, and it is built from factors, never from guesswork. Step through each example one line at a time; the bins on the left show exactly which factors each denominator already owns, and which ones it is missing.
The factor bins
LCD—
Excluded values—
factor this denominator has factor it lacks what you must multiply by
Build the LCD, one line at a timepick an example below