# Substitute an equation

**URL:** <https://discourse.julialang.org/t/substitute-an-equation/62913>\
**Category:** General Usage\
**Tags:** question, symbolic, symbolics\
**Created:** [June 14, 2021, 9:30pm UTC](https://discourse.julialang.org/t/substitute-an-equation/62913 "2021-06-14T21:30:25Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![feanor12](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/feanor12/32/8212_2.png) [@feanor12](https://discourse.julialang.org/u/feanor12)\
**Post date:** [June 14, 2021, 9:30pm UTC](https://discourse.julialang.org/t/substitute-an-equation/62913/1 "2021-06-14T21:30:25Z")

</div>

If I have two equations how do I combine them?

Do I have to write something like this or is there a shorter way? Maybe something like `solve(eq1,eq2,a)`?

```julia
using Symbolics
@vars a b c
eq1 = ( a = b + c)
eq2 = ( b = 3c - a)
substitute(eq1,eq2.lhs => eq2.rhs)

```

---

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [June 15, 2021, 6:17am UTC](https://discourse.julialang.org/t/substitute-an-equation/62913/2 "2021-06-15T06:17:12Z")

</div>

Are you looking for `solve_for`?

---

<div class="post-metadata">

**Author:** ![feanor12](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/feanor12/32/8212_2.png) [@feanor12](https://discourse.julialang.org/u/feanor12)\
**Post date:** [June 15, 2021, 6:48am UTC](https://discourse.julialang.org/t/substitute-an-equation/62913/3 "2021-06-15T06:48:20Z")

</div>

Perfect! How did I miss this.

---

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [June 15, 2021, 7:15am UTC](https://discourse.julialang.org/t/substitute-an-equation/62913/4 "2021-06-15T07:15:30Z")

</div>

It was wearing a fake mustache.