For this problem you just plug in the number for the variable.
Heart Rate = 65 + 53/(4t + 1)
Heart Rate = 65 + 53/(4*10 + 1)
Heart Rate = 65 + 53/41
Heart Rate = 66.29
If we know heart rate is 68 we can state that
68 = 65 + 53/(4t + 1)
solve for t
3 = 53/(4t + 1)
3(4t+1)=53
12t + 3 = 53
12t = 53
t = 4.16 minutes