When tom found the difference -) he got -15. what might tom have done wrong

He did -11 - 4 which is -15. He subtracted 4 from -11 when he should subtract -4 from -11. Subtracting -4 is the same as adding 4.

The problem is -11 - (-4) which is the same as -11 + 4 = -7.

b. an equilateral triangle inscribed in a circle

step-by-step explanation:

we are given the figure of two circles having a triangle inside.

now, we know that,

"when a figure is lying inside an another figure, then the outside figure is known as circumscribed figure and the figure lying inside is referred to as the inscribed figure".

as we see that, the circles are containing the triangle completely.

thus, the circles are the circumscribed figures and the triangle is the inscribed figure.

hence, we get, 'an equilateral triangle is inscribed in a circle'.