There's no new maths here — just the three facts from rung 1, used in reverse. Instead of checking the total, you already know the total and you're hunting the one piece that's missing.
The Three Moves
One — name the fact. Look at the picture. Angles sharing a straight edge? That's 180°. Angles fanning out all the way round a point? That's 360°. An X-cross where x sits opposite a known angle? They're equal.
Two — write the equation. Put the known angles and x into a sum that equals the right total. For a straight line: known + x = 180.
Three — solve for x. Subtract the knowns from the total. x = 180 − known. Done.
A Worked One
Two angles sit on a straight line. One is 65°, the other is x. The fact is "straight line → 180°", so 65 + x = 180, which gives x = 180 − 65 = 115°. For an "around a point" question with angles of 120° and 95°: 120 + 95 + x = 360, so x = 360 − 215 = 145°.
Always Write the Reason
In an exam you don't just write the number — you write the fact you used, like "angles on a straight line sum to 180°". That one line is worth marks, and it forces you to check you picked the right rule before you commit. Get into the habit now.