HELP with channels

Hello
i need to plot channels around my price based on the following formula :

Upper Channel Line = EMA + EMA * Channel Coefficient
Lower Channel Line = EMA - EMA * Channel Coefficient

where,
the EMA and Channel Coefficient can be changed manually..

I am new to amibroker so i dont know if this is a silly question or what.. but hope to get a solution..

Thanks in advance..

periodEMA = param( "periodEMA", 10, 3, 30, 1);
channelCoef = param("channel coefficient", 1, .1, 10, .1 );

upperchannel = ...
lowerchannel = ...

plot(c, "", color, stylebar);
plot(upperchannel, ...;
plot(lowerchannel, ...