Logic geometry. Ah, textbooks.
Flip dat triangle!
Turn in onto its base.
My Geometry teacher used to say, "My students, ahh. Textbooks. They say, 'My pretty little students, here's a formula! I will give you numbers and you plug it in.' Then he would say, 'I don't like textbooks. I'm a rebel! So, I derive the formula.'" He made us derive it. But the formula is this: 1/2 x b x h. Why? The real formula is because two equal triangles are both HALF of a parallelogram, and a parallelogram has formula b x h. So it's 1/2 x b x h.
With that in mind, let's find the height first. Onwards!!
12 mm. What can we do with it? Oh, we know the rectangle has 8 mm!! 12 mm - 8 mm is 4 mm! The height is 4 mm. Onwards to the base!!
Hey, what do we know? The rectangle has length 10 mm. A part of it is 4 mm!! So, what's 10 mm - 4 mm? 6 mm. We got it!!
Plug in the formula!!
1/2 x 4 x 6 is????????
12.
And don't forget the mm!!