How to convolve $u(-t)$ with other signals?
In general, convolution of a function $f(t)$ with $u(-t)$ results in a function $g(t)$ defined by the following integral:

$$g(t)=\int_{-\infty}^{\infty}f(\tau)u(\tau-t)d\tau=\int_{t}^{\infty}f(\tau)d\tau\tag{1}$$

In the given example, $f(t)=u(t+1)$, so you get

$$g(t)=\int_{t}^{\infty}u(\tau+1)d\tau=\int_{t+1}^{\infty}u(\tau)d\tau\tag{2}$$

I leave it up to you to figure out what the result of that integral is.