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