Derivation of Gravitational Potential

Hi,

The derivation of the Gravitational Potential formula, as I understand, is:

[itex] W = Fd [/itex] (1)

[itex] W = G \frac{M_1m_2}{r^2}d [/itex] (2) Substituting the Gravitational Force formula

[itex] W = - \int_R^∞G \frac{M_1m_2}{r^2} \, dr [/itex] (3) Integrating within the boundaries of the initial distance (R) and Infinity

Which allows us to arrive at:

[itex] E_p = - \frac{GM_2m_1}{R}[/itex] (4)

However, what I don't understand is how we are able to proceed from step 3 to step 4.
What method must be used in order to proceed as such?

My proficiency with Calculus is still in the works.

Thanks,