Step-by-step explanation:
a. Area of a circle = πr², where r means radius.
Step 1: converted diameter to radius.
diameter = 2 radius
In other way of writing, radius =  diameter.
 diameter.
Step 2: substitute radius into the formula.
Area of a circle = π( d)²
d)²
Step 3: substitute π into the formula. 
some question is given the value of π), if not substitute 3.14 for π.
Area of a circle = 3.14( d)²
d)²
Step 4: substitute d into the formula, then we can solve its area.
b. a. Circumference of a circle = 2πr, where r means radius.
Step 1: converted diameter to radius.
diameter = 2 radius
In other way of writing, radius =  diameter.
 diameter.
Step 2: substitute radius into the formula.
C = 2π( d)
d)
Step 3: substitute π into the formula. 
some question is given the value of π), if not substitute 3.14 for π.
C = 2(3.14)( d)
d)
Step 4: substitute d into the formula, then we can solve its circumference.