Alright, so you know that the formula for the area of a triangle is a=bh/2
First start out by identifying your base and height.
Base is the side that the shape is "standing on", in this case, that would be 9cm.
Now, the height is the measurement that is perpendicular (90 degrees to the base), which in this case is 12 cm.
Now, plug these values into your formula
(12)(9) /2 = 54 cm squared! hope this helps :)