Q:

What is the slave address of the STM32 NUCLEO-F401RE?

I am currently trying to learn I2C communications with two STM32 NUCLEO-F401RE boards and I am having trouble on how to figure out the slave address of the board. I searched the datasheets and the internet but could not find it and I'm not sure how to determine it. 

A:

You should have searched the Reference Manual of the controller. But as said the MCU's I2C address can be set from software through dedicated registers.
These are called Own Address Register 1 and 2. For the most simple 7-bit addressing mode you have to clear the ADDMODE bit and set any kind of address you want using ADD[7:1] bits.
Here are the relevant register descriptions but I suggest to read through the entire I2C chapter of the linked document.

