2x+8 is the same as 2*x+8. This is basically saying "multiply 2 times some number, then add on 8". The order of operation PEMDAS says that multiplication comes before addition.
The x is a placeholder for a value. In this case, it is -7 as we go from f(x) to f(-7). All we've done is replace x with -7
f(x) = 2*x+8
f(-7) = 2*(-7)+8 ... replace every x with -7
f(-7) = -14+8
f(-7) = -6
Answer: -6