Curtis is twice as old as his brother jonathan. olivia is six years younger than jonathan. the sum of their ages is 58 years.
an equation .

C=2 times j
c=2j
o=j-6
c+j+o=58

subsitte j-6 for o and 2j for c

2j+j+j-6=58
4j-6=58
add 6 to both sides
4j=64
divide both sides by 4
j=16

sub back

c=2j
c=2(16)
c=32

o=j-6
o=16-6
o=10

curtis=32
olivia=10
jonathan=16

equations are
c+j+o=58
c=2j
o=j-6

If you have Jonathan's age (J), you can figure out the rest by adding the other two parts to the equation, that being Olivia (O) being six years younger than Johnathan (subtraction), and Curtis (C) being twice Johnathan's age (multiplication.) 'Course, you need to know the value of a variable to actually figure it out.
In other words
 O = J - 6
and
C = J x 2
So the equation might look like:
J + J(2) + (J-6) = 58