Generally, "when" is used to find algebraic expressions or function values from known unknown values.

For example, given x=5, find the value of y=2x,

You can say: when x=5, y=2*5= 10.

You can get the answer directly with "namely".

For example, AB=5 and AC=2AB are known.

You can say: AB = 5, AC=2AB.

∴AC= 10 (in other words, AC= 10)