The variable is "d".
It is: d
The coefficient would be the number multiplied by the variable, in this case: 7
The coefficient of 3+7d is 7
7 is the coefficient of the variable d
7d + 2 7 is the only coefficient
what is the constant in algebraic experssion 7d+2
d is the variable here.
The expression simplified is: 7d-2cd-5c
15d + 7 = 7d + 15 is an equation in one variable, d.