7x + 2y = 48 , when y = 3?

Well you already know that y=3 so just substitute.

7x+2y=48 > 7x+2(3)=48 > 7x+6=48 > 7x=42 > x=6

So the answer is 6.

Step by step:

Step 1: Substitute 3 for y and get 7x+2(3)=48
Step 2: Multiply 3 by 2 and get 6 in order to eliminate excess terms
Step 3: Subtract 6 from 48 and get 42 to isolate the x term
Step 4: Divide 42 by 7 to complete the isolation of the x term
Step 5: x=6 :)

Hydrophobia (fear of or aversion to water).

Objective pronoun is the answer.