2x+c=d and 2x+1=9 are similar in arithmetic structure, but the first += equation substitutes algebraic symbols for the numbers. If we take the algebraic form and make x the subject we get 2x=d-c, so x=(d-c)/2.
Now, if we substitute for c and d (c=1 and d=9), x=(9-1)/2, x=8/2=4.