First, do not use the exponentiation operator here:
y1 = (2*exp(-2*t)-7*exp(-2*t)+0.6)/(4*exp^(-2*t));
↑
and use element-wise multiplication (.*) and division (./) operations:
y1 = (2*exp(-2*t)-7.*exp(-2*t)+0.6)./(4*exp(-2*t));
↑ ↑
Do the same with all of them, and remember the parentheses in the denominator on the others.