[tex]y=0.25[/tex] or [tex] \frac{1}{4}[/tex]; you first need to plug in the [tex]x[/tex] value, making the equation [tex]7(3)-4y=20[/tex]. Multiply [tex]7[/tex] and [tex]3[/tex] to get [tex]21[/tex] and then subtract it from [tex]20[/tex] in order to isolate the [tex]y[/tex]. This results in [tex]-4y=-1[/tex]. Then divide both sides by [tex]-4[/tex] and the answer would be [tex] \frac{1}{4} [/tex], since dividing two numbers that have the same sign results in a positive number and [tex] \frac{1}{4} [/tex] can not be simplified any further, unless you are looking for the answer in decimal form, in which [tex]y=0.25[/tex].
