You will have to add the 2 sums together, and then calculate the income tax on the total sum. I've done some calculations for you below.
Contrary to what many people think, the state pension is not tax-free, but the money you receive is paid 'gross' - without any tax being deducted. If your total income from all sources, including the state pension, is greater than your tax-free allowance, tax is due on your state pension and this will normally be deducted from any private pension or earnings you might have, which are paid through the PAYE system.
So here are your calculations:
£2100 a month from pension is £25200 a year
Your husband's £55K + £25200 = £80200 gross earnings a year.
If you run a tax calculation on this, this is what you'll get:
Take home (net): £4,524 every month
Tax break-down:
You'll pay no tax on the first £11,500 that you're earning.
You'll pay £6,700 tax on £33,500 (at 20%)
You'll pay £14,080 tax on £35,200 (at 40%)
Total tax paid: £20,780
See here: www.income-tax.co.uk/after-tax/80200/
I hope it helps.