On this question for example

7a + 8b = 17 (1)

4a + 6b = 9 (2)

I mutilply the coeffecient of b in equation (2) to the top equation and get

42a + 48b = 102

then multiply the coeffecient of b in equation (1) to the bottom equation

32a + 48b = 72

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the b gets cancelled out so then I subtract right? 42a - 32a = 10a

102 - 72 = 30

so does that mean 10a gets subtracted from 30 to leave a = 20?

Is this the correct method for this type of question? If not could someone perhaps go over where I've gone wrong. Thanks for your help.