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How to evaluate an expression that contains function notation (example)

Sal evaluates -2*f(-6)+g(1) given the graphs of f and g. Created by Sal Khan.
Video transcript
We’re asked to evaluate -2 × f(-6) + g(1). And they've defined – at least graphically – f(x) and g(x) here below. So let’s see how we can evaluate this. Well, to do this, we first have to figure out what f(-6) is. So our input into our function is -6. So we'll assume that's along the horizontal axis. So our input axis is -6. And, based on our function definition, f(-6) is 7. So this thing – Let me write this down. f(-6 ) = 7. And what is g(1)? Well, once again, here's our input axis. And then the function says that g(1) – which is right over there – is -5 g(1) = -5. So this statement simplifies to -2 × f(-6), which is 7 ... so times 7 + g(1), which is -5 ... so + (-5) … which simplifies to – let's see ... -2 ×7 is -14 plus -5, which is -19. And we are done.