Multi variable problem

Question

If a - b= -6 and b+c= 9 find the value of 3a^2 - b^2 -2c^2

Answer 1

a - b = -6, so a = b - 6

b + c = 9, so c = 9 - b

3a² - b² - 2c²

Substitute b-6 for a and 9-b for c:

3(b - 6)² - b² - 2(9 - b)²

Multiply it out and combine like terms to get the answer.