Micheal and susan have a combined age of 17 years. micheal is 5 years older than susan

Let x be the unknown number which is the age of Susan

Susan's age: x
Michael's age: x + 5

Both total 17 years. The equation would be.

x + x + 5 = 17
2x + 5 = 17

Find x. Transfer 5 to the other side and change its sign
2x = 17 - 5
2x = 12

Find x. Divide both sides by 2.
2x / 2 = 12 / 2
x = 6 => Susan's Age.

Michael's age: x + 5 => substitute x by its corresponding value: 6 + 5 = 11.

To check.

Susan's Age + Michael's age = 17
6 + 11 = 17
17 = 17