N=1e3

#error term and regressor
eps=rnorm(N,0,9)
x=rnorm(N)

#starting value
y0=0

y=c(y0,rep(NA,N-1))

for(i in 2:N)
{
  y[i] = 0.75*y[i-1]+7*x[i]+eps[i]
}

for(i in 2:N)
{
  y[i] = 0.75*y[i-1]+eps[i]
}

y=y+7*x