# Puzzle on a blackboard [closed]

What is the missing number in the picture above?(question is from some puzzle game) I tried some approaches, but haven't found any meaningful rule..thanks in advance!

• Welcome to Puzzling! What puzzle game is this from? If a question is taken from somewhere else, you need to specify where exactly it's from. – Deusovi Jun 30 '19 at 0:50

$$\frac{\frac{(x-9)\cdot 5}2-4+12}3+7=x$$
$$\frac56x+\frac{13}6=x$$
$$x=13.$$