the main thing for here is to combine the like terms and to solve for n which is done by putting all the numbers with variables on one side and the non variables on the other and divide each side by the number that has the variable. Remember that when you bring numbers on the other side, you change its sign, for example if you have 5 on one side and you bring to the other side of the equal sign it becomes -5.
-15 + 15d -7.5d =7.5
15d - 7.5d = 7.5 + 15
7.5d = 22.5
d = 3
for the part where theres 7.5(2d-1)
you multiply each number inside the bracket by 7.5, its called distributivity of multiplication
