What is the area of a triangle length 2.5 height 8.75

Area of a triangle = 0.5 x base x height
                            = 0.5 x 2.5 x 8.75
                            = 10.94 (units squared) 

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. 

Parallelogram area formula is b x h, or base x height. So half of that? 
(b x h) / 2, or 1/2 x b x h.

Plug this in. 

Assuming your triangle is equilateral, I plug in 2.5 for the base and 8.75 for the height. 

So, what's 1/2 x 2.5 x 8.75?

It's 10.9375.

Rounded, it's 10.94.

You have it.

Just remember, it's about WHY this formula works.