and you also don't know the velocity here.

你还不知道这里的速度
Remember, you have to solve for the velocity,

你得解出速度
and you have to solve for the m'.

你得解出 m'
If you say what are the unknowns and what are the knowns,

如果你想知道什么是未知什么是已知
let's be very clear.

我们就来搞清楚
The photon has a known momentum and known energy.

光子具有已知的动量和能量
That's given to you.

这个已经给出了
Initial me has a mass m that's given to you.

我的初质量 m 已经给出
Your job is to find the unknown mass m' and the unknown velocity.

你的任务是找出未知质量 m' 和速度
Two equations for two unknowns,

两个方程两个未知数
but they're not simple linear equations where you can add and subtract.

但它们不是可以直接加减的简单线性方程
The simple linear equations are multiply this by three,

简单的线性方程是 3 乘这个