-g + 3/4a = y for a

[math_celebrity]

-g + 3/4a = y for a

Add g to each side:
-g + g + 3/4a = y + g

Cancel the g terms on the left side:
3/4a = y + g

Cross multiply:
3a = 4(y + g)

Divide each side by 3 to isolate a:
3a/3 = 4(y + g)/3
a = 4(y + g)/3