Final answer:
The y-intercept in Sally's earnings equation for babysitting, which is the money she receives not dependent on the hours worked, is $18.
Step-by-step explanation:
The y-intercept of the equation that represents Sally's earnings from babysitting for the Smiths every Friday can be determined by looking at the fixed amount she receives, which occurs when she works 0 hours (x = 0). Since the problem states that she gets paid $18 for food, this amount is provided regardless of the hours worked, which makes it the y-intercept. Thus, the value representing the y-intercept is $18 (a).