f(x)=2x+c, g(x) = cx+5, fg(x)= 6x+d, work out the value of d

Let’s call (f(x)=2x+c) equation 1, (g(x) = cx+5) equation 2 and (fg(x)= 6x+d) equation 3.
Start by finding fg(x) in terms of c by substituting (equation 2) into (equation 1) to get (fg(x)= 2(cx +5) + c). You can then equate this with (equation 3) and expand to get 2cx +10 + c= 6x+d. We can’t know what the value of x is but we can equate the two coefficients of x, meaning 2c=6, therefore c=3. ‘d’ represents the rest of the terms on the left-hand side of the equation, meaning that d= 10+c. Since c=3, d=10+3, therefore d=13.