A mixture of oxygen, hydrogen, and nitrogen exerts a total pressure of 378 kPa. If the partial pressures of oxygen and hydrogen are 212 kPa and 101 kPa respectively, what is the partial pressure exerted by nitrogen?

What's the relationship between total and partial pressure? The total pressure is the sum of the parcial pressures!

So for us, it would be:

378 = 212+101+x

where x is the parcial pressure of nitrogen.

Now we count:

378 = 212+101+x

378=313+x

378-313=x

65=x

So the parcial pressure exerted by nitrogen is 65!