Calculating the Charge of a Capacitor
It's a fundamental equation governing capacitors.  

$$q(t) = C \cdot v(t)$$

You just have to understand what exactly \$q\$ and \$v\$ represent. \$v(t)\$ here is the voltage across the plates of the capacitor. When there is a resistor involved, you have to solve the circuit for V. If you haven't got any calculus under your belt that is not trivial, as capacitors have complex (frequency-dependent) impedance.