\(\color{red}(3x+4)(x-1)\)
= \(\color{orange}3x(x-1)+4(x-1)\)<--- Multiplication Distributive Law
=\(\color{lime}(3x)(x)-(3x)(1)+4x-4\) <--- Multiplication Distributive Law again
= \(\color{blue}3x^2-3x+4x-4\)<--- Multiplying out the brackets.
= \(\color{purple}3x^2+x-4\)<--- Combining like terms. Finish.