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.