To the third power? The expansion of ax+b cubed is a^3x^3+3a^2x^2b+3axb^2+b^3. So in the question a=7 and b=-3/4. Also, x is replaced by y. The expansion is therefore f(y)=343y-(441/4)y^2+(189/16)y-27/64. In decimals this is 343x^3-110.25x^2+11.8125x-0.421875. There is no equation to solve, it's just the definition of a particular function.