With #1. it would be deeply appreciated.

Your equation is a good place to start. Let x represent the measure of the angle.

... (1/2)(90 -x) +21 = x

... 90 -x +42 = 2x . . . . . multiply by 2

... 132 = 3x . . . . . . . . . . collect terms, add x

... 44 = x . . . . . . . . . . . . divide by 3. This is the measure of the angle.

180 - 44 = 136

The supplement of the angle is 136°.

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'.